fix: stabilize streamed message rendering

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/src/Eryx.AgentHost/Contracts/ProtocolModels.cs

@@ -167,6 +167,7 @@ public sealed class TurnDeltaEventDto : SidecarEventDto
     public string MessageId { get; init; } = string.Empty;
     public string AuthorName { get; init; } = string.Empty;
     public string ContentDelta { get; init; } = string.Empty;
+    public string? Content { get; init; }
 }
 
 public sealed class TurnCompleteEventDto : SidecarEventDto

sidecar/src/Eryx.AgentHost/Services/CopilotWorkflowRunner.cs

@@ -119,6 +119,7 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
                 string messageId = update.Update.MessageId ?? $"{command.RequestId}-delta-{fallbackMessageIndex++}";
                 StreamingSegment segment = GetOrCreateSegment(segments, messageId, authorName);
                 segment.SetContent(StreamingTextMerger.Merge(segment.Content.ToString(), update.Update.Text));
+                segment.SetAuthorName(authorName);
 
                 await onDelta(new TurnDeltaEventDto
                 {
@@ -128,6 +129,7 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
                     MessageId = messageId,
                     AuthorName = authorName,
                     ContentDelta = update.Update.Text,
+                    Content = segment.Content.ToString(),
                 }).ConfigureAwait(false);
             }
             else if (evt is ExecutorCompletedEvent completed
@@ -477,7 +479,7 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
 
         public string MessageId { get; }
 
-        public string AuthorName { get; }
+        public string AuthorName { get; private set; }
 
         public StringBuilder Content { get; } = new();
 
@@ -486,6 +488,11 @@ public sealed class CopilotWorkflowRunner : ITurnWorkflowRunner
             Content.Clear();
             Content.Append(value);
         }
+
+        public void SetAuthorName(string value)
+        {
+            AuthorName = value;
+        }
     }
 
     private sealed class AgentBundle : IAsyncDisposable

sidecar/tests/Eryx.AgentHost.Tests/SidecarProtocolHostTests.cs

@@ -128,6 +128,7 @@ public sealed class SidecarProtocolHostTests
                     MessageId = "assistant-1",
                     AuthorName = "Primary",
                     ContentDelta = "Hello",
+                    Content = "Hello",
                 });
 
                 await onActivity(new AgentActivityEventDto
@@ -201,6 +202,7 @@ public sealed class SidecarProtocolHostTests
             {
                 Assert.Equal("turn-delta", deltaEvent.GetProperty("type").GetString());
                 Assert.Equal("Hello", deltaEvent.GetProperty("contentDelta").GetString());
+                Assert.Equal("Hello", deltaEvent.GetProperty("content").GetString());
             },
             toolEvent =>
             {