.NET: Track and update A2A task state

[SergeyMenshykh]

Motivation & Context

A2A hosting can return either a message or a task. Previously, when it returned a background task, subsequent agent updates were not applied, leaving the task indefinitely in the Working state.

This change uses the agent streaming API for both response types. The server either aggregates updates into a message, returns an initial task and continues updating it, or waits for the stream to finish and returns the completed task.

Description & Review Guide

• What are the major changes? New A2A messages now run through the streaming agent API. Background-enabled registrations either stream task updates after returning the initial task or aggregate them into a completed task, depending on ReturnImmediately. Background-disabled registrations aggregate the stream into a message. The task aggregation path also preserves response metadata on artifacts, and the four server/client response combinations have dedicated coverage.
• What is the impact of these changes? A2A tasks receive status and artifact updates instead of remaining indefinitely in Working state, while message responses and non-immediate task responses retain their expected response shapes.
• What do you want reviewers to focus on? Please focus on the response routing across AgentRunMode and ReturnImmediately, and on task artifact construction in the aggregate path.

Related Issue

Fixes #5362

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Updates A2A hosting to stream agent responses into task state and artifacts.

Changes:

• Routes new messages through streaming execution.
• Supports immediate streaming and aggregated task/message responses.
• Adds coverage for response modes, empty responses, and metadata.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AAgentHandler.cs	Implements streaming and task aggregation.
dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/A2AAgentHandlerTests.cs	Tests routing and task lifecycle behavior.
dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/A2AServerServiceCollectionExtensionsTests.cs	Adds end-to-end server response tests.

Suppressed comments (1)

dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AAgentHandler.cs:102

• Routing the non-streaming endpoint through this helper drops the run-mode value that the previous path passed as AgentRunOptions.AllowBackgroundResponses: the shared helper calls CreateRunOptions(context) without the result of _runMode. Implementations such as FoundryHostedRequestAgent and ChatClientAgent use this option to activate background/resumable execution, so AllowBackgroundIfSupported now only wraps a foreground run in a task and DisallowBackground no longer explicitly disables it. Evaluate the run mode before creating the options and pass that result into the streaming run.

        await this.HandleNewMessageStreamingAsync(context, eventQueue, aggregateTaskUpdates, cancellationToken).ConfigureAwait(false);

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[github-actions]

MAF Automated Review — Iteration 1

Result: Findings reported
Scope: full PR (1 commit(s)): b811d678ea46
Model: gpt-5.6-sol

Overview

The review found 4 verified inline finding(s).

Reviewed the supplied pull-request change set across correctness, security/reliability, architecture, and failure behavior.
4 verified findings remained after source verification (1 high, 3 medium) across 1 file. Details are attached to the affected lines below.

Affected areas: dotnet/src/Microsoft.Agents.AI.Hosting.A2A/A2AAgentHandler.cs

[github-actions]

MAF Automated Review — Iteration 2

Result: No findings
Scope: 2 net-new commit(s): a123466c41cf, c6f388123935
Model: gpt-5.6-sol-fast

Overview

This incremental change closes the previously reported post-submission lifecycle gap by transitioning aggregated tasks to Canceled or Failed with an uncancelled token when result emission fails. The extracted artifact helper preserves the existing event shape, and focused tests exercise both cancellation and serialization failure after submission. No new Critical, High, or Medium defect was established in the authoritative incremental range.

Reviewed the supplied incremental change set across correctness, security/reliability, architecture, and failure behavior.
No publishable findings remained after source verification for this scope.

[Roger Barreto (rogerbarreto)]

Suggest renaming this method as this is not a streaming-only behavior and now has more responsibility.

Suggested change

	private async Task HandleNewMessageStreamingAsync(RequestContext context, AgentEventQueue eventQueue, bool aggregateTaskUpdates, CancellationToken cancellationToken)
	private async Task HandleNewMessageCoreAsync(RequestContext context, AgentEventQueue eventQueue, bool aggregateTaskUpdates, CancellationToken cancellationToken)