feat(a2a): add A2AAgentSession with reference_task_ids and input-required support (#5980)

* feat(a2a): link follow-up messages via reference_task_ids

Track the task_id from A2A responses (task, status_update, artifact_update,
and message payloads) on session.state and include it as reference_task_ids
on subsequent outgoing messages. This enables remote agents to correlate
follow-up messages as task refinements per the A2A spec.

Resolves #5938

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

* feat(a2a): add A2AAgentSession for typed protocol state tracking

Introduce A2AAgentSession (subclass of AgentSession) with context_id,
task_id, and task_state properties. This follows the DurableAgentSession
pattern and mirrors the .NET A2AAgentSession design.

- Track task_id, context_id, and task_state from all response payload types
- Validate context_id consistency (raise on mismatch)
- Auto-assign server-generated context_id when not set
- Only A2AAgentSession gets reference tracking (no state dict fallback)
- Plain AgentSession continues to work without reference tracking
- Add serialization support (to_dict/from_dict)
- Export via agent_framework.a2a and agent_framework_a2a

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

* style: remove unnecessary string annotation (pyupgrade)

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

* fix: use AgentSession.from_dict for state deserialization

Avoids importing private _deserialize_state, matching the
DurableAgentSession pattern.

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

* fix: track context_id from message payloads in A2AAgentSession

Previously, context_id was only captured from task, status_update, and
artifact_update payloads. Message-only responses (which carry context_id
but may lack task_id) were silently lost. This fix:

- Captures msg.context_id in the message handler
- Persists session state when either last_task_id or last_context_id is
  present (not only when task_id is truthy)
- Only updates task_id/task_state when a task_id was actually returned
- Adds a test for message-only context_id tracking

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

* addressed comments

* Gate status content to INPUT_REQUIRED/terminal states (match .NET)

Match .NET's GetUserInputRequests pattern: only emit TaskStatusUpdateEvent
message content when state is INPUT_REQUIRED or terminal. Intermediate
status text (WORKING, SUBMITTED) is no longer surfaced to callers.

When state is INPUT_REQUIRED, set additional_properties['input_required']
= True so callers can distinguish input requests from final responses.

Closes #5937

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

* Address review: remove message task_id tracking, defensive fallbacks, and input_required flag

- Do not track task_id from Message payloads (simple interactions
  without task tracking)
- Remove 'or last_task_id' fallback from status_update and
  artifact_update handlers (spec guarantees task_id is always set)
- Remove additional_properties['input_required'] flag (content gating
  to INPUT_REQUIRED/terminal states is the signal itself)

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

---------

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

python/packages/core/agent_framework/a2a/__init__.py

@@ -7,6 +7,7 @@ This module lazily re-exports objects from:
 
 Supported classes:
 - A2AAgent
+- A2AAgentSession
 - A2AExecutor
 """
 
@@ -15,7 +16,7 @@ from typing import Any
 
 IMPORT_PATH = "agent_framework_a2a"
 PACKAGE_NAME = "agent-framework-a2a"
-_IMPORTS = ["A2AAgent", "A2AExecutor"]
+_IMPORTS = ["A2AAgent", "A2AAgentSession", "A2AExecutor"]
 
 
 def __getattr__(name: str) -> Any:

python/packages/core/agent_framework/a2a/__init__.pyi

@@ -1,5 +1,5 @@
 # Copyright (c) Microsoft. All rights reserved.
 
-from agent_framework_a2a import A2AAgent, A2AExecutor
+from agent_framework_a2a import A2AAgent, A2AAgentSession, A2AExecutor
 
-__all__ = ["A2AAgent", "A2AExecutor"]
+__all__ = ["A2AAgent", "A2AAgentSession", "A2AExecutor"]