.NET: Fix stream reconnection for A2AAgent (#5275)

* Add SSE stream reconnection support to A2AAgent

Implement automatic reconnection for SSE streams that disconnect mid-task,
using the Last-Event-ID header to resume from where the stream left off.

Changes:
- Add InvokeStreamingWithReconnectAsync method to A2AAgent with configurable
  max retries and delay between attempts
- Add new log messages for reconnection events
- Add A2AAgent_StreamReconnection sample demonstrating the feature
- Update existing polling sample to use simplified SendMessageAsync API
- Add unit tests for stream reconnection logic

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

* address comments

* Address PR review feedback

- Dispose SSE enumerator before GetTaskAsync fallback to release HTTP connection
- Wrap StreamWriter in using blocks with leaveOpen:true and explicit UTF-8 encoding
- Print update.Text instead of update object in stream reconnection sample

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

---------

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

dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs

@@ -155,7 +155,7 @@ public sealed class A2AAgent : AIAgent
 
         if (GetContinuationToken(messages, options) is { } token)
         {
-            streamEvents = this._a2aClient.SubscribeToTaskAsync(new SubscribeToTaskRequest { Id = token.TaskId }, cancellationToken).ConfigureAwait(false);
+            streamEvents = this.SubscribeToTaskWithFallbackAsync(token.TaskId, cancellationToken).ConfigureAwait(false);
         }
         else
         {
@@ -248,6 +248,75 @@ public sealed class A2AAgent : AIAgent
         return typedSession;
     }
 
+    /// <summary>
+    /// Subscribes to task updates, falling back to <see cref="A2AClient.GetTaskAsync"/>
+    /// when the task has already reached a terminal state and the server responds with
+    /// <see cref="A2AErrorCode.UnsupportedOperation"/>.
+    /// </summary>
+    /// <remarks>
+    /// Per A2A spec §3.1.6, subscribing to a task in a terminal state (completed, failed,
+    /// canceled, or rejected) results in an <c>UnsupportedOperationError</c>.
+    /// See: <see href="https://a2a-protocol.org/latest/specification/#332-error-handling"/>.
+    /// </remarks>
+    private async IAsyncEnumerable<StreamResponse> SubscribeToTaskWithFallbackAsync(
+        string taskId,
+        [EnumeratorCancellation] CancellationToken cancellationToken)
+    {
+        var subscribeStream = this._a2aClient.SubscribeToTaskAsync(new SubscribeToTaskRequest { Id = taskId }, cancellationToken);
+
+        var enumerator = subscribeStream.GetAsyncEnumerator(cancellationToken);
+
+        // yield return cannot appear inside a try block that has catch clauses,
+        // so we manually advance the enumerator within try/catch and yield outside it.
+        // The outer try/finally (no catch) is allowed to contain yield return in C#.
+        StreamResponse? fallbackResponse = null;
+        bool disposed = false;
+
+        try
+        {
+            while (true)
+            {
+                bool hasNext;
+                try
+                {
+                    hasNext = await enumerator.MoveNextAsync().ConfigureAwait(false);
+                }
+                catch (A2AException ex) when (ex.ErrorCode == A2AErrorCode.UnsupportedOperation)
+                {
+                    this._logger.LogA2ASubscribeToTaskFallback(this.Id, this.Name, taskId, ex.Message);
+
+                    // Dispose the enumerator before the fallback call to release the HTTP/SSE connection.
+                    await enumerator.DisposeAsync().ConfigureAwait(false);
+                    disposed = true;
+
+                    AgentTask agentTask = await this._a2aClient.GetTaskAsync(new GetTaskRequest { Id = taskId }, cancellationToken).ConfigureAwait(false);
+
+                    fallbackResponse = new StreamResponse { Task = agentTask };
+                    break;
+                }
+
+                if (!hasNext)
+                {
+                    break;
+                }
+
+                yield return enumerator.Current;
+            }
+
+            if (fallbackResponse is not null)
+            {
+                yield return fallbackResponse;
+            }
+        }
+        finally
+        {
+            if (!disposed)
+            {
+                await enumerator.DisposeAsync().ConfigureAwait(false);
+            }
+        }
+    }
+
     private static void UpdateSession(A2AAgentSession? session, string? contextId, string? taskId = null)
     {
         if (session is null)
@@ -321,17 +390,17 @@ public sealed class A2AAgent : AIAgent
         };
     }
 
-    private AgentResponse ConvertToAgentResponse(AgentTask agentTask)
+    private AgentResponse ConvertToAgentResponse(AgentTask task)
     {
         return new AgentResponse
         {
             AgentId = this.Id,
-            ResponseId = agentTask.Id,
-            FinishReason = MapTaskStateToFinishReason(agentTask.Status.State),
-            RawRepresentation = agentTask,
-            Messages = agentTask.ToChatMessages() ?? [],
-            ContinuationToken = CreateContinuationToken(agentTask.Id, agentTask.Status.State),
-            AdditionalProperties = agentTask.Metadata?.ToAdditionalProperties(),
+            ResponseId = task.Id,
+            FinishReason = MapTaskStateToFinishReason(task.Status.State),
+            RawRepresentation = task,
+            Messages = task.ToChatMessages() ?? [],
+            ContinuationToken = CreateContinuationToken(task.Id, task.Status.State),
+            AdditionalProperties = task.Metadata?.ToAdditionalProperties(),
         };
     }
 
@@ -360,6 +429,7 @@ public sealed class A2AAgent : AIAgent
             RawRepresentation = task,
             Role = ChatRole.Assistant,
             Contents = task.ToAIContents(),
+            ContinuationToken = CreateContinuationToken(task.Id, task.Status.State),
             AdditionalProperties = task.Metadata?.ToAdditionalProperties(),
         };
     }

dotnet/src/Microsoft.Agents.AI.A2A/A2AAgentLogMessages.cs

@@ -34,4 +34,17 @@ internal static partial class A2AAgentLogMessages
         string methodName,
         string agentId,
         string? agentName);
+
+    /// <summary>
+    /// Logs <see cref="A2AAgent"/> falling back to GetTaskAsync after SubscribeToTaskAsync failed with UnsupportedOperation.
+    /// </summary>
+    [LoggerMessage(
+        Level = LogLevel.Warning,
+        Message = "A2AAgent {AgentId}/{AgentName} SubscribeToTask for task '{TaskId}' failed with UnsupportedOperation: {ErrorMessage}. Falling back to GetTaskAsync.")]
+    public static partial void LogA2ASubscribeToTaskFallback(
+        this ILogger logger,
+        string agentId,
+        string? agentName,
+        string taskId,
+        string errorMessage);
 }