Add streaming support to A2A agent handler

Add HandleNewMessageStreamingAsync to A2AAgentHandler that routes
StreamingResponse requests through RunStreamingAsync, enqueuing an A2A
Message for each AgentResponseUpdate.

Add MessageConverter.ToParts(AgentResponseUpdate) extension to convert
streaming update contents to A2A Parts with unsupported-content filtering.

Add CreateMessageFromUpdate to map AgentResponseUpdate to A2A Message.

Add 16 new tests covering the streaming path and converter.

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

Author: SergeyMenshykh

dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AAgentHandler.cs

@@ -42,11 +42,19 @@ public A2AAgentHandler(
     /// <inheritdoc/>
     public Task ExecuteAsync(RequestContext context, AgentEventQueue eventQueue, CancellationToken cancellationToken)
     {
+        // Handle task updates
         if (context.IsContinuation)
         {
             return this.HandleTaskUpdateAsync(context, eventQueue, cancellationToken);
         }
 
+        // Handle messages received via streaming endpoint
+        if (context.StreamingResponse)
+        {
+            return this.HandleNewMessageStreamingAsync(context, eventQueue, cancellationToken);
+        }
+
+        // Handle new messages received via non-streaming endpoint
         return this.HandleNewMessageAsync(context, eventQueue, cancellationToken);
     }
 
@@ -108,6 +116,34 @@ private async Task HandleNewMessageAsync(RequestContext context, AgentEventQueue
         }
     }
 
+    private async Task HandleNewMessageStreamingAsync(RequestContext context, AgentEventQueue eventQueue, CancellationToken cancellationToken)
+    {
+        var contextId = context.ContextId ?? Guid.NewGuid().ToString("N");
+        var session = await this._hostAgent.GetOrCreateSessionAsync(contextId, cancellationToken).ConfigureAwait(false);
+
+        // AIAgent does not support resuming from arbitrary prior tasks.
+        // Throw explicitly so the client gets a clear error rather than a response
+        // that silently ignores the referenced task context.
+        if (context.Message?.ReferenceTaskIds is { Count: > 0 })
+        {
+            throw new NotSupportedException("ReferenceTaskIds is not supported. AIAgent cannot resume from arbitrary prior task context.");
+        }
+
+        List<ChatMessage> chatMessages = context.Message is not null ? [context.Message.ToChatMessage()] : [];
+
+        var options = context.Metadata is { Count: > 0 }
+            ? new AgentRunOptions { AdditionalProperties = context.Metadata.ToAdditionalProperties() }
+            : null;
+
+        await foreach (var update in this._hostAgent.RunStreamingAsync(chatMessages, session, options, cancellationToken).ConfigureAwait(false))
+        {
+            var message = CreateMessageFromUpdate(contextId, update);
+            await eventQueue.EnqueueMessageAsync(message, cancellationToken).ConfigureAwait(false);
+        }
+
+        await this._hostAgent.SaveSessionAsync(contextId, session, cancellationToken).ConfigureAwait(false);
+    }
+
     private async Task HandleTaskUpdateAsync(RequestContext context, AgentEventQueue eventQueue, CancellationToken cancellationToken)
     {
         var contextId = context.ContextId ?? Guid.NewGuid().ToString("N");
@@ -174,6 +210,16 @@ private static Message CreateMessageFromResponse(string contextId, AgentResponse
             Metadata = response.AdditionalProperties?.ToA2AMetadata()
         };
 
+    private static Message CreateMessageFromUpdate(string contextId, AgentResponseUpdate update) =>
+        new()
+        {
+            MessageId = update.ResponseId ?? Guid.NewGuid().ToString("N"),
+            ContextId = contextId,
+            Role = Role.Agent,
+            Parts = update.ToParts(),
+            Metadata = update.AdditionalProperties?.ToA2AMetadata()
+        };
+
     private static List<ChatMessage> ExtractChatMessagesFromTaskHistory(AgentTask? agentTask)
     {
         if (agentTask?.History is not { Count: > 0 })

dotnet/src/Microsoft.Agents.AI.Hosting.A2A/Converters/MessageConverter.cs

@@ -8,6 +8,26 @@ namespace Microsoft.Agents.AI.Hosting.A2A.Converters;
 
 internal static class MessageConverter
 {
+    public static List<Part> ToParts(this AgentResponseUpdate update)
+    {
+        if (update.Contents is not { Count: > 0 })
+        {
+            return [];
+        }
+
+        var parts = new List<Part>();
+        foreach (var content in update.Contents)
+        {
+            var part = content.ToPart();
+            if (part is not null)
+            {
+                parts.Add(part);
+            }
+        }
+
+        return parts;
+    }
+
     public static List<Part> ToParts(this IList<ChatMessage> chatMessages)
     {
         if (chatMessages is null || chatMessages.Count == 0)