fix: use mTLS transport for outbound A2A calls to Google endpoints

[ShresthSamyak]

Summary

A2A remote calls made via AgentRegistry.get_remote_a2a_agent(...) fail with
401 UNAUTHENTICATED when the orchestrator uses Agent Identity, while
working fine with a standard Service Account. This routes outbound A2A traffic
to Google endpoints over a mutual-TLS channel so channel-bound credentials are
accepted.

Fixes #6365.

Root cause

get_remote_a2a_agent() builds a RemoteA2aAgent with only an optional
httpx_client, and RemoteA2aAgent creates a plain httpx.AsyncClient for
the actual A2A RPCs. Google context-aware access policies (used by Agent
Identity) issue access tokens that are cryptographically bound to a mutual-TLS
channel; presenting a bound token over a plain connection is rejected with
401 UNAUTHENTICATED.

The mTLS work in 03671c6 hardened only the AgentRegistry control-plane calls
(_make_request via AuthorizedSession). The A2A data-plane client never
carried the client certificate or a channel-bound token, so:

Identity	Token	Over plain client	Result
Service Account	unbound bearer	accepted anywhere	works
Agent Identity	mTLS/DPoP channel-bound	binding check fails	401

get_mcp_toolset() already injects Google auth for Google MCP endpoints;
get_remote_a2a_agent() had no equivalent for A2A. This aligns them.

Changes

• New utils/_async_mtls_transport.py — the async google-auth mTLS
  transport (_RefreshableAsyncCredentials, _GoogleAuthAsyncTransport,
  _SharedAsyncTransport, _GoogleAuthAsyncByteStream) extracted from
  mcp_session_manager.py, plus a create_google_auth_mtls_transport(url)
  factory. Shared by both the MCP and A2A paths.
• mcp_session_manager.py — imports the transport classes from the shared
  module instead of defining them locally. No behavior change.
• remote_a2a_agent.py — new enable_google_auth_mtls flag; when set and
  no httpx_client is supplied, the default client is built on the mTLS
  transport, falling back to a plain client if mTLS cannot be negotiated.
• agent_registry.py — enables the flag for Google A2A endpoints when a
  client certificate is configured and GOOGLE_API_USE_MTLS_ENDPOINT != "never".
  A caller-supplied httpx_client is never overridden; if it would suppress
  required mTLS, a warning is logged so the 401 is diagnosable.

Behavior / compatibility

• Non-Google endpoints and Service Account flows are unaffected.
• A user-provided httpx_client is always respected (with a warning when it
  would suppress required mTLS).
• The mTLS path requires google.auth.aio (google-auth[aiohttp]); if
  unavailable, it degrades gracefully to a plain client.
• No public API removed; enable_google_auth_mtls is an additive,
  default-False parameter.

Testing plan

Unit tests (pytest), all passing (230 passed):

• tests/unittests/utils/test_async_mtls_transport.py — factory success /
  non-mTLS / exception / no-aio paths, and token host-scoping.
• tests/unittests/integrations/agent_registry/test_agent_registry.py — mTLS
  enabled for Google endpoints; not enabled for non-Google, without a client
  cert, with a caller-supplied client, or when the endpoint is set to never;
  and that the suppressed-mTLS warning fires (and stays silent for non-Google).
• tests/unittests/agents/test_remote_a2a_agent.py — default client is plain
  when disabled, uses the mTLS transport when enabled, and falls back when mTLS
  is unavailable.
• tests/unittests/tools/mcp_tool/test_mcp_session_manager.py — unchanged MCP
  mTLS behavior after the extraction.

Additionally verified locally that create_google_auth_mtls_transport()
negotiates a real mutual-TLS channel and presents the client certificate to a
server that requires it, with the bearer token attached over that channel
(returns 200; a cert-less client is rejected at the handshake).

Live validation against a deployed Agent Runtime + Agent Identity setup is
pending; the success path emits
Successfully configured mTLS using AsyncAuthorizedSession at INFO.

[ShresthSamyak]

Thanks for taking a look, @rohityan, and for merging main in.

I've pushed two follow-up commits to address the CI failures from this change:

• e0157c9 — fixes the mypy and compliance checks. For mypy, I added __all__ to the new _async_mtls_transport module so AsyncAuthorizedSession is treated as an explicit re-export, and a # type: ignore on the google.auth.aio import fallback shim. For compliance, the new tests were using incidental googleapis.com hostnames, which I changed to example.com.
• 3bebd5e — removes a stray trailing blank line in .agents/skills/adk-style/references/imports.md that was tripping the end-of-file-fixer hook.

A few of the remaining red checks appear unrelated to this PR's diff, but please let me know if I'm missing something:

• Unit Tests: the failures are all test_verify_snippets.py erroring at collection with FileNotFoundError for open_source_workspace/.agents/skills/adk-verify-snippets/scripts/verify_md.py. That path looks specific to the internal checkout layout, so it may not resolve on the public runner — the rest of the suite passes.
• Mypy: the one remaining flagged line is in models/lite_llm.py, which this PR doesn't modify.
• Do Not Merge on GitHub / Copybara: the standard external-PR marker.

Happy to make any adjustments you'd like on the fix itself. Thanks again!