fix(a2a) warn when session state is dropped

[sylvesterkaczmarek]

Link to Issue or Description of Change

• Closes Session state does not cross a RemoteA2aAgent boundary in either direction, silently #6854

Problem

Session state is local to each side of a RemoteA2aAgent boundary, but state loss was silent. A caller state-only event contributes no A2A parts, so the peer receives older content. Remote state deltas are intentionally rejected because a peer must not mutate caller state, but that rejection was only debug-logged.

Solution

• Warn before an outbound state-only hand-off is dropped.
• Warn when an inbound remote state delta is rejected.
• Document the boundary in the API docstring and task-mode guide.
• Document safe alternatives: send peer inputs as content and return caller-visible values as content or task output.

This preserves the security boundary while making both loss directions visible.

Testing Plan

• Added regression tests.
• Relevant tests pass.

pytest tests/unittests/agents/test_remote_a2a_agent.py tests/unittests/a2a/converters/test_to_adk.py -q --disable-warnings

262 passed in 2.56s

Both warning directions are covered and unsafe state mutation remains blocked.

All applicable pre-commit hooks passed.

Manual E2E

Not run against deployed A2A services. The tests exercise the exact request-history and inbound-metadata boundaries.

Checklist

• I have read the CONTRIBUTING document.
• I have self-reviewed this change.
• I have added regression tests.
• Relevant tests pass locally.
• I have tested against deployed A2A services.
• This change has no downstream dependencies.

Additional context

State forwarding remains out of scope because applying peer-controlled state would cross the existing trust boundary.

[iarjunganesh]

Thanks for addressing the runtime boundary here. I traced the converter path and
confirmed that the inbound warning covers a remote LlmAgent(output_key=...):
the remote event's stateDelta is serialized into artifact metadata and then
rejected by the caller converter with the new warning.

I opened #6862 for the complementary caller-side construction hazard: a local
LlmAgent(output_key=...) immediately followed by RemoteA2aAgent. That event
also contains content, so the state-only outbound warning in this PR does not
fire even though the associated session key is unavailable remotely. #6862 is
limited to that detectable ordering and touches separate files.

One review point for this PR: _construct_message_parts_from_session() checks
ctx.session.events[-1] before task-scope filtering. In task mode, that event
can be outside the current isolation scope, which could warn for an event that
is not part of the actual hand-off. Would it be safer to inspect the last
applicable event after events_to_process is built?

It may also be useful to add a round-trip regression test that starts with an
ADK event containing an output_key state delta, converts it to an A2A
artifact, and verifies that the caller emits the warning while preserving the
response content. That would directly lock down Direction 1 from #6854.