refactor: drop unused MessageHeader.StripLocalHeaders instance method

No transport calls it — they all use IsLocalHeader for inline filtering
or BagWithoutLocalHeaders for whole-bag JSON serialisation. Keeping the
method around invites callers to mutate the message's bag in place,
which would re-introduce the InMemoryOutbox-by-reference regression that
moving stripping out of the mediator just fixed.

Test class renamed MessageHeaderStripLocalHeadersTests →
MessageHeaderLocalHeadersTests and the two strip-specific cases rewritten
against BagWithoutLocalHeaders (also asserts the original header is
untouched, pinning the InMemoryOutbox-by-reference invariant).

Co-Authored-By: Claude (claude-opus-4-7) <noreply@anthropic.com>

Author: DevJonny

src/Paramore.Brighter/MessageHeader.cs

@@ -134,18 +134,6 @@ public static void RegisterLocalHeader(string name)
             }
         }
 
-        /// <summary>
-        /// Removes every local-header entry (see <see cref="IsLocalHeader"/>) from
-        /// <see cref="Bag"/>. Call on a wire-bound copy of the header to ensure
-        /// local-only bag entries are not serialised onto the transport.
-        /// </summary>
-        public void StripLocalHeaders()
-        {
-            var locals = Volatile.Read(ref s_localHeaderNames);
-            foreach (var name in locals)
-                Bag.Remove(name);
-        }
-
         /// <summary>
         /// Returns a new dictionary containing every <see cref="Bag"/> entry whose key
         /// is not a local header (see <see cref="IsLocalHeader"/>). For transports that

tests/Paramore.Brighter.Core.Tests/MessageSerialisation/When_Stripping_Local_Headers_From_A_Message_Header.cs

@@ -2,10 +2,10 @@
 
 namespace Paramore.Brighter.Core.Tests.MessageSerialisation;
 
-public class MessageHeaderStripLocalHeadersTests
+public class MessageHeaderLocalHeadersTests
 {
     [Fact]
-    public void When_Stripping_Local_Headers_The_ProducerTopic_Bag_Entry_Is_Removed_And_Other_Entries_Survive()
+    public void When_BagWithoutLocalHeaders_Removes_Local_Entries_And_Other_Entries_Survive()
     {
         var header = new MessageHeader(
             messageId: "id-1",
@@ -14,25 +14,27 @@ public void When_Stripping_Local_Headers_The_ProducerTopic_Bag_Entry_Is_Removed_
         header.Bag[Message.ProducerTopicHeaderName] = "lookup.topic";
         header.Bag["user.key"] = "user.value";
 
-        header.StripLocalHeaders();
+        var wireBag = header.BagWithoutLocalHeaders();
 
-        Assert.False(header.Bag.ContainsKey(Message.ProducerTopicHeaderName));
-        Assert.True(header.Bag.ContainsKey("user.key"));
+        Assert.False(wireBag.ContainsKey(Message.ProducerTopicHeaderName));
+        Assert.True(wireBag.ContainsKey("user.key"));
+        // original is untouched — InMemoryOutbox-by-reference keeps the local entry for retries
+        Assert.True(header.Bag.ContainsKey(Message.ProducerTopicHeaderName));
     }
 
     [Fact]
-    public void When_Stripping_Local_Headers_With_No_Local_Bag_Entries_Then_The_Bag_Is_Unchanged()
+    public void When_BagWithoutLocalHeaders_With_No_Local_Entries_Returns_Equivalent_Copy()
     {
         var header = new MessageHeader(
             messageId: "id-2",
             topic: new RoutingKey("a.topic"),
             messageType: MessageType.MT_EVENT);
         header.Bag["user.key"] = "user.value";
 
-        header.StripLocalHeaders();
+        var wireBag = header.BagWithoutLocalHeaders();
 
-        Assert.Single(header.Bag);
-        Assert.Equal("user.value", header.Bag["user.key"]);
+        Assert.Single(wireBag);
+        Assert.Equal("user.value", wireBag["user.key"]);
     }
 
     [Fact]
@@ -44,7 +46,7 @@ public void When_The_Producer_Topic_Header_Name_Is_A_Local_Header()
     [Fact]
     public void When_RegisterLocalHeader_Adds_A_Custom_Key_It_Is_Recognised_And_Is_Idempotent()
     {
-        const string customKey = "custom.local.header." + nameof(MessageHeaderStripLocalHeadersTests);
+        const string customKey = "custom.local.header." + nameof(MessageHeaderLocalHeadersTests);
 
         MessageHeader.RegisterLocalHeader(customKey);
         MessageHeader.RegisterLocalHeader(customKey); // idempotent