fix(tests): resolve flaky FailedJob_CanBeRetried test

  Replace fixed Task.Delay(200) calls with proper synchronization using
  TaskCompletionSource signals. The test was failing on slow CI machines
  because 200ms wasn't enough time for job processing to complete before
  the retry was enqueued.

  The handler now exposes FirstInvocationCompleted and SecondInvocationCompleted
  tasks that the test awaits with a 5-second timeout, making the test
  deterministic regardless of machine speed.

Author: Ibrahim5aad

tests/Octopus.Server.Processing.Tests/ProcessingWorkerServiceTests.cs

@@ -270,7 +270,10 @@ public async Task FailedJob_CanBeRetried()
             Version = 1
         };
         await queue.EnqueueAsync(envelope1);
-        await Task.Delay(200);
+
+        // Wait for first invocation to complete (with timeout)
+        var firstCompleted = await Task.WhenAny(handler.FirstInvocationCompleted, Task.Delay(5000));
+        Assert.True(firstCompleted == handler.FirstInvocationCompleted, "First invocation did not complete within timeout");
 
         // Retry the same job
         var envelope2 = new JobEnvelope
@@ -282,7 +285,10 @@ public async Task FailedJob_CanBeRetried()
             Version = 1
         };
         await queue.EnqueueAsync(envelope2);
-        await Task.Delay(200);
+
+        // Wait for second invocation to complete (with timeout)
+        var secondCompleted = await Task.WhenAny(handler.SecondInvocationCompleted, Task.Delay(5000));
+        Assert.True(secondCompleted == handler.SecondInvocationCompleted, "Second invocation did not complete within timeout");
 
         cts.Cancel();
         queue.Complete();
@@ -313,13 +319,28 @@ public class FailingThenSucceedingHandler : IJobHandler<TestJobPayload>
         public string JobType => "TestJob";
         public int InvocationCount { get; private set; }
 
+        private readonly TaskCompletionSource _firstInvocation = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        private readonly TaskCompletionSource _secondInvocation = new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        /// <summary>
+        /// Completes when the first invocation has finished (after throwing).
+        /// </summary>
+        public Task FirstInvocationCompleted => _firstInvocation.Task;
+
+        /// <summary>
+        /// Completes when the second invocation has finished (successfully).
+        /// </summary>
+        public Task SecondInvocationCompleted => _secondInvocation.Task;
+
         public Task HandleAsync(string jobId, TestJobPayload payload, CancellationToken cancellationToken = default)
         {
             InvocationCount++;
             if (InvocationCount == 1)
             {
+                _firstInvocation.TrySetResult();
                 throw new InvalidOperationException("Simulated failure on first attempt");
             }
+            _secondInvocation.TrySetResult();
             return Task.CompletedTask;
         }
     }