feat(a2a): Add optional context_builder to RemoteA2aAgent

[a2105z]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Configurable Context Builder for RemoteA2aAgent #3301

Problem:
RemoteA2aAgent always builds remote A2A request context via a fixed _construct_message_parts_from_session path (full/partial session history with "For context:" formatting). That is inflexible and token-heavy for specialized remotes (current-turn only, sliding window, summarization), and the only escape hatch today is monkey-patching the protected method.

Solution:
Add an optional context_builder argument to RemoteA2aAgent.__init__, typed as a public ContextBuilder protocol:

class ContextBuilder(Protocol):
  def __call__(
      self,
      ctx: InvocationContext,
      agent_name: str,
      genai_part_converter: GenAIPartToA2APartConverter,
  ) -> tuple[list[A2APart], str | None]: ...

When provided, it fully replaces default session→message construction for the outgoing request. When None, behavior is unchanged (_construct_message_parts_from_session).

This matches the API requested in #3301 exactly.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

pytest tests/unittests/agents/test_remote_a2a_agent.py -q
# 158 passed

Coverage added for:

• storing context_builder on init
• custom builder used (default constructor not called)
• fallback when context_builder is None
• _run_async_impl path uses the custom builder

Manual End-to-End (E2E) Tests:

• Not run against a live remote A2A server in this PR (unit coverage exercises the request-construction seam). Happy to follow up with a sample if useful.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Took this over because #3301 had no open PR and no recent implementation activity (earlier interest note from May did not land a patch). Implemented exactly the optional ContextBuilder protocol + fallback described in the issue.

cc @klateefa — thought I’d take this over given no open PR / nothing in flight. Please redirect if you’d rather keep design internal or if someone already has a patch.

[a2105z]

Hi @DeanChensj @GWeale @Jacksunwei — hoping one of you might have a minute for a review when free.

This is the fix for #3301: an optional public context_builder on RemoteA2aAgent so callers can customize outgoing A2A request context (current-turn only, sliding window, summarization, etc.) without monkey-patching _construct_message_parts_from_session. Default path is unchanged when context_builder is omitted.

Covered in tests/unittests/agents/test_remote_a2a_agent.py (158 passed locally). Happy to adjust the Protocol / call signature if you’d prefer a different shape — thanks so much for taking a look.

[a2105z]

Hi @DeanChensj — quick follow-up: I don’t have triage rights to edit labels on this repo. Could you please add a2a (and needs review if that’s useful) alongside the existing core label? Thanks!