fix(a2a): warn when output_key cannot reach remote session

[iarjunganesh]

Link to Issue or Description of Change

• Related: Session state does not cross a RemoteA2aAgent boundary in either direction, silently #6854
• Complements: fix(a2a) warn when session state is dropped #6859

Problem:

When an LlmAgent with output_key immediately precedes a
RemoteA2aAgent in a SequentialAgent, the generated content crosses A2A but
the associated session-state key does not. Because the event also has content,
the state-only outbound warning proposed in #6859 does not cover this hand-off.

Solution:

Warn at SequentialAgent construction for that specific adjacent ordering.
The check detects an already-loaded RemoteA2aAgent without importing it, so
ordinary SequentialAgent users do not acquire a dependency on the optional
A2A SDK.

Testing Plan

Unit Tests:

• Added positive coverage for LlmAgent(output_key) immediately followed
  by RemoteA2aAgent.
• Added negative coverage for reversed ordering, a remote hand-off without
  output_key, and an entirely local hand-off.
• pytest tests/unittests/agents/test_sequential_agent.py -q
  (10 passed on Python 3.12).
• Strict mypy passed for sequential_agent.py.
• pyink, isort, ruff, ADK compliance checks, and codespell passed
  for both changed files.
• Verified in an isolated Python 3.12 environment without a2a-sdk that
  importing and constructing a local SequentialAgent still works.

Manual End-to-End (E2E) Tests:

Not run. This change is a deterministic construction-time warning and does not
alter execution or transport behavior.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented the optional-dependency-sensitive code.
• I have added tests that prove the warning and no-warning paths.
• New and existing tests in the affected unit pass locally.
• I have manually tested the change end-to-end.
• This change has no downstream dependencies.

Additional context

This is intentionally separate from #6859: that PR covers state-only outbound
events and state deltas received from a remote peer. This PR covers a
caller-side output_key event that also contains content, so it is neither a
duplicate nor dependent on #6859.

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