Python: preserve A2A message context_id

[Eduard van Valkenburg (eavanvalkenburg)]

Motivation and Context

This change fixes #4679 by preserving the context_id attached to outbound A2A messages, which keeps multi-turn A2A conversations correlated correctly.

Description

Forward context_id from the framework Message into the generated A2AMessage and add a regression test that verifies _prepare_message_for_a2a() preserves it.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR fixes #4679 by preserving the A2A context_id when converting framework Message objects to outbound A2AMessage, keeping multi-turn A2A conversations correctly correlated.

Changes:

• Forward context_id from Message.additional_properties["context_id"] into the generated A2AMessage.context_id.
• Add a regression unit test to ensure _prepare_message_for_a2a() preserves context_id.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
python/packages/a2a/agent_framework_a2a/_agent.py	Adds forwarding of context_id into outbound A2A message construction.
python/packages/a2a/tests/test_a2a_agent.py	Adds regression coverage for context_id preservation in _prepare_message_for_a2a().

---

You can also share your feedback on Copilot code review. Take the survey.

[Mark Wallace (markwallace-microsoft)]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/a2a/agent_framework_a2a
_agent.py	184	13	92%	383, 404, 425, 445, 459, 473, 485–486, 528–529, 558–560
TOTAL	23919	2627	89%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
5219	20 💤	0 ❌	0 🔥	1m 21s ⏱️