Python: propagate A2A status message ids

[he-yufeng]

Fixes #5949.

A2A status updates already map the A2A task id to AgentResponseUpdate.response_id, but the status message id was not carried into AgentResponseUpdate.message_id. This keeps that mapping for both streamed TaskStatusUpdateEvent messages and in-progress task status messages.

To verify

• uv run --frozen pytest tests\test_a2a_agent.py -q from python/packages/a2a
• uv run --frozen ruff check agent_framework_a2a_agent.py tests\test_a2a_agent.py from python/packages/a2a
• uv run --frozen ruff format --check agent_framework_a2a_agent.py tests\test_a2a_agent.py from python/packages/a2a
• python -m py_compile python\packages\a2a\agent_framework_a2a_agent.py python\packages\a2a\tests\test_a2a_agent.py
• git diff --check

[Copilot]

Pull request overview

This PR fixes the A2A→Agent Framework identifier mapping for status updates by propagating A2A message_id into AgentResponseUpdate.message_id (in addition to the existing mapping of A2A task_id → AgentResponseUpdate.response_id). This applies to both streamed TaskStatusUpdateEvent messages and in-progress task status messages surfaced during streaming.

Changes:

• Populate AgentResponseUpdate.message_id for in-progress task status messages emitted via _updates_from_task.
• Populate AgentResponseUpdate.message_id for streamed TaskStatusUpdateEvent updates emitted via _updates_from_task_update_event.
• Extend tests to assert the correct message_id propagation for both paths.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/a2a/agent_framework_a2a/_agent.py	Adds propagation of A2A status message_id into AgentResponseUpdate.message_id for task/status streaming updates.
python/packages/a2a/tests/test_a2a_agent.py	Updates mocks and assertions to validate message_id propagation for in-progress and status-update-event streaming.