Keep OnFault as the name for saga fault transitions

An earlier commit on this branch renamed OnFault to OnReplyFault because it
only covers fault replies. That encoded a coverage gap into the name.

The extensions layer names concepts over the channel-level primitives of the
core descriptor: OnTimeout is OnRequest<SagaTimedOutEvent> and is not called
OnRequestTimeout. OnFault belongs to that vocabulary, and not covering publish
is a missing feature rather than a naming boundary.

The narrower name also survives neither plausible future. If publish faults are
delivered as a NotAcknowledgedEvent on the reply endpoint, the same route and
the same transition catch them and OnReplyFault becomes too narrow. If fan-in
semantics warrant a separate hook, the symmetric pair is OnSendFault and
OnPublishFault, where a channel name sits oddly beside a verb name.

The scope is documented instead. Neither a failing subscriber of a published
event nor a failing saga transition produces a reply, so neither reaches this
transition, and the summary now says so.

This removes the public API rename from the branch: OnFault keeps the signature
it has on main, and only its body and documentation change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: alisan3

src/Mocha/src/Examples/Sagas/ParallelSaga/ParallelSaga.cs

@@ -171,7 +171,7 @@ protected override void Configure(ISagaDescriptor<ReturnSagaState> descriptor)
         // Fault during inspection: compensate by marking failed
         descriptor
             .During(AwaitingInspection)
-            .OnReplyFault()
+            .OnFault()
             .Then((state, fault) => state.FailureReason = $"Inspection failed: {fault.ErrorMessage}")
             .TransitionTo(Failed);
 
@@ -207,7 +207,7 @@ protected override void Configure(ISagaDescriptor<ReturnSagaState> descriptor)
         // Fault while waiting for either reply: compensation
         descriptor
             .During(AwaitingBothReplies)
-            .OnReplyFault()
+            .OnFault()
             .Then((state, fault) => state.FailureReason = $"Parallel step failed: {fault.ErrorMessage}")
             .TransitionTo(Failed);
 

src/Mocha/src/Mocha/Sagas/Descriptors/SagaStateDescriptorExtensions.cs

@@ -9,13 +9,14 @@ namespace Mocha.Sagas;
 public static class SagaStateDescriptorExtensions
 {
     /// <summary>
-    /// Registers a transition triggered by a fault reply to a message the saga dispatched. It does not
-    /// cover failures of events the saga published.
+    /// Registers a transition triggered by a fault reply to a message the saga sent. It covers neither
+    /// failures of events the saga published nor failures of the saga's own transitions, since neither
+    /// produces a reply.
     /// </summary>
     /// <typeparam name="TState">The saga state type.</typeparam>
     /// <param name="descriptor">The state descriptor to configure.</param>
     /// <returns>A descriptor for configuring the fault transition.</returns>
-    public static ISagaTransitionDescriptor<TState, NotAcknowledgedEvent> OnReplyFault<TState>(
+    public static ISagaTransitionDescriptor<TState, NotAcknowledgedEvent> OnFault<TState>(
         this ISagaStateDescriptor<TState> descriptor)
         where TState : SagaStateBase
     {
@@ -24,7 +25,7 @@ public static ISagaTransitionDescriptor<TState, NotAcknowledgedEvent> OnReplyFau
 
     /// <summary>
     /// Registers a transition triggered by any successful reply. A state that declares this must also
-    /// handle fault replies through <c>OnReplyFault</c>, either on the state itself or through
+    /// handle fault replies through <c>OnFault</c>, either on the state itself or through
     /// <c>DuringAny</c>.
     /// </summary>
     /// <typeparam name="TState">The saga state type.</typeparam>

src/Mocha/src/Mocha/Sagas/Initialization/SagaValidator.cs

@@ -108,6 +108,6 @@ private static void ValidateFaultHandling(Saga saga, string stateName, SagaState
         throw new SagaInitializationException(
             saga,
             $"State '{stateName}' handles any reply but does not handle faults. "
-            + "Add '.OnReplyFault()' to this state, or '.DuringAny().OnReplyFault()' to the saga.");
+            + "Add '.OnFault()' to this state, or '.DuringAny().OnFault()' to the saga.");
     }
 }

src/Mocha/test/Mocha.Sagas.Tests/IntegrationTests.cs

@@ -312,10 +312,10 @@ public async Task RpcReply_Should_NotSelectSaga_When_SameResponseTypeRequestedDi
     }
 
     [Fact]
-    public async Task Saga_Should_ReceiveFault_When_SendUsedWithOnReplyFault()
+    public async Task Saga_Should_ReceiveFault_When_SendUsedWithOnFault()
     {
         // A saga that uses .Send to dispatch a request whose handler fails terminally routes the
-        // fault reply back to its OnReplyFault transition, correlated by the saga header the fault reply
+        // fault reply back to its OnFault transition, correlated by the saga header the fault reply
         // carries. Without it the saga waits in its send state until it times out.
 
         // arrange
@@ -336,7 +336,7 @@ public async Task Saga_Should_ReceiveFault_When_SendUsedWithOnReplyFault()
         // assert - the handler ran and threw, proving the request was delivered
         await handlerFaulted.Task.WaitAsync(s_timeout, TestContext.Current.CancellationToken);
 
-        // assert - the fault routed back to the saga and drove its OnReplyFault transition
+        // assert - the fault routed back to the saga and drove its OnFault transition
         var fault = await FaultSaga.Observed.Task.WaitAsync(s_timeout, TestContext.Current.CancellationToken);
         Assert.Equal(ErrorCodes.Exception, fault.ErrorCode);
     }
@@ -472,7 +472,7 @@ protected override void Configure(ISagaDescriptor<RequestResponseState> descript
                 .TransitionTo("AwaitingResponse");
 
             descriptor.During("AwaitingResponse").OnAnyReply().TransitionTo("Completed");
-            descriptor.During("AwaitingResponse").OnReplyFault().TransitionTo("Completed");
+            descriptor.During("AwaitingResponse").OnFault().TransitionTo("Completed");
 
             descriptor.Finally("Completed");
         }
@@ -495,15 +495,15 @@ protected override void Configure(ISagaDescriptor<RequestResponseState> descript
                 .TransitionTo("AwaitingResponse");
 
             descriptor.During("AwaitingResponse").OnAnyReply().TransitionTo("Completed");
-            descriptor.During("AwaitingResponse").OnReplyFault().TransitionTo("Completed");
+            descriptor.During("AwaitingResponse").OnFault().TransitionTo("Completed");
 
             descriptor.Finally("Completed");
         }
     }
 
     /// <summary>
     /// Fault saga: StartFaultEvent -> AwaitingResponse (sends FaultingRequest) ->
-    /// OnReplyFault -> Failed (final)
+    /// OnFault -> Failed (final)
     /// </summary>
     public sealed class FaultSaga : Saga<RequestResponseState>
     {
@@ -520,7 +520,7 @@ protected override void Configure(ISagaDescriptor<RequestResponseState> descript
 
             descriptor
                 .During("AwaitingResponse")
-                .OnReplyFault()
+                .OnFault()
                 .Then((_, fault) => Observed.TrySetResult(fault))
                 .TransitionTo("Failed");
 

src/Mocha/test/Mocha.Sagas.Tests/ReplyDiagnosticsTests.cs

@@ -129,7 +129,7 @@ protected override void Configure(ISagaDescriptor<DiagnosticsState> descriptor)
                 .Send((_, _) => new DiagnosticsRequest())
                 .TransitionTo("Awaiting");
 
-            descriptor.During("Awaiting").OnReplyFault().TransitionTo("Failed");
+            descriptor.During("Awaiting").OnFault().TransitionTo("Failed");
 
             descriptor.Finally("Failed");
         }
@@ -147,7 +147,7 @@ protected override void Configure(ISagaDescriptor<DiagnosticsState> descriptor)
                 .TransitionTo("Awaiting");
 
             descriptor.During("Awaiting").OnAnyReply().TransitionTo("Done");
-            descriptor.During("Awaiting").OnReplyFault().TransitionTo("Done");
+            descriptor.During("Awaiting").OnFault().TransitionTo("Done");
 
             descriptor.Finally("Done");
         }

src/Mocha/test/Mocha.Sagas.Tests/SagaPublishFaultTests.cs

@@ -152,7 +152,7 @@ protected override void Configure(ISagaDescriptor<PublishState> descriptor)
                 .Publish((_, state) => new ErrorQueueEvent(state.Id))
                 .TransitionTo("Awaiting");
 
-            descriptor.During("Awaiting").OnReplyFault().TransitionTo("Failed");
+            descriptor.During("Awaiting").OnFault().TransitionTo("Failed");
 
             descriptor.Finally("Failed");
         }
@@ -186,7 +186,7 @@ protected override void Configure(ISagaDescriptor<PublishState> descriptor)
 
             descriptor
                 .During("Awaiting")
-                .OnReplyFault()
+                .OnFault()
                 .Then((_, fault) => s_faultObserved.TrySetResult(fault))
                 .TransitionTo("Failed");
 

src/Mocha/test/Mocha.Sagas.Tests/SagaRouteConditionTests.cs

@@ -26,7 +26,7 @@ public void Configure_Should_GateOnSagaIdOnly_When_OnAnyReply()
     }
 
     [Fact]
-    public void Configure_Should_GateOnSagaIdAndMessageType_When_OnReplyFault()
+    public void Configure_Should_GateOnSagaIdAndMessageType_When_OnFault()
     {
         // arrange & act
         var runtime = CreateRuntime(b => b.AddSaga<AnyReplySaga>());
@@ -122,7 +122,7 @@ protected override void Configure(ISagaDescriptor<ReplyState> descriptor)
                 .TransitionTo("Awaiting");
 
             descriptor.During("Awaiting").OnAnyReply().TransitionTo("Done");
-            descriptor.During("Awaiting").OnReplyFault().TransitionTo("Done");
+            descriptor.During("Awaiting").OnFault().TransitionTo("Done");
 
             descriptor.Finally("Done");
         }

src/Mocha/test/Mocha.Sagas.Tests/SagaSendCommandTests.cs

@@ -128,7 +128,7 @@ protected override void Configure(ISagaDescriptor<CommandState> descriptor)
 
             descriptor
                 .During("Awaiting")
-                .OnReplyFault()
+                .OnFault()
                 .Then((_, fault) => s_faultObserved.TrySetResult(fault))
                 .TransitionTo("Failed");
 
@@ -153,7 +153,7 @@ protected override void Configure(ISagaDescriptor<CommandState> descriptor)
                 .Then((_, reply) => s_replyObserved.TrySetResult(reply))
                 .TransitionTo("Done");
 
-            descriptor.During("Awaiting").OnReplyFault().TransitionTo("Done");
+            descriptor.During("Awaiting").OnFault().TransitionTo("Done");
 
             descriptor.Finally("Done");
         }

src/Mocha/test/Mocha.Sagas.Tests/SagaStateMachineTests.cs

@@ -415,7 +415,7 @@ public async Task Saga_Should_Fallback_When_AnyEvent()
 
                 x.During("Started").OnAnyReply().TransitionTo("Triggered");
 
-                x.During("Started").OnReplyFault().TransitionTo("Ended");
+                x.During("Started").OnFault().TransitionTo("Ended");
 
                 x.During("Triggered").OnEvent<End>().TransitionTo("Ended");
 

src/Mocha/test/Mocha.Sagas.Tests/SagaValidationTests.cs

@@ -189,7 +189,7 @@ public void Initialize_Should_Throw_When_StateHandlesAnyReplyWithoutFault()
         // assert
         Assert.Equal(
             "State 'Awaiting' handles any reply but does not handle faults. "
-            + "Add '.OnReplyFault()' to this state, or '.DuringAny().OnReplyFault()' to the saga.",
+            + "Add '.OnFault()' to this state, or '.DuringAny().OnFault()' to the saga.",
             exception.Message);
     }
 
@@ -205,7 +205,7 @@ public void Initialize_Should_PassValidation_When_StateHandlesAnyReplyAndFault()
                 .TransitionTo("Awaiting")
                 .StateFactory(_ => new TestState(Guid.NewGuid(), "Awaiting"));
             descriptor.During("Awaiting").OnAnyReply().TransitionTo("Success");
-            descriptor.During("Awaiting").OnReplyFault().TransitionTo("Success");
+            descriptor.During("Awaiting").OnFault().TransitionTo("Success");
             descriptor.Finally("Success");
         });
 
@@ -227,7 +227,7 @@ public void Initialize_Should_PassValidation_When_FaultHandledByDuringAny()
                 .TransitionTo("Awaiting")
                 .StateFactory(_ => new TestState(Guid.NewGuid(), "Awaiting"));
             descriptor.During("Awaiting").OnAnyReply().TransitionTo("Success");
-            descriptor.DuringAny().OnReplyFault().TransitionTo("Success");
+            descriptor.DuringAny().OnFault().TransitionTo("Success");
             descriptor.Finally("Success");
         });