fix(a2a): preserve late reply waiters and reject undelivered finals

[RuniThomsen]

Summary

• Keep an admitted task waiting when its observation deadline passes; elapsed observation time is not proof that the running agent failed.
• Log the elapsed window once with task/context correlation.
• Return an unsuccessful SendResult and an ERROR when a final has no pending waiter, instead of silently discarding it with success=True.

Verification

Both regressions were observed failing on the original implementation before their fixes. After rebasing onto main deb4bd2:
scripts/run_tests.sh tests/plugins/test_a2a_late_reply.py tests/plugins/test_a2a_plugin.py tests/plugins/test_a2a_phase23.py tests/plugins/test_a2a_tools_gate.py
Result: 142 passed, 0 failed across four files.

Scope and limitations

No credential, identity, configuration or runtime activation changes. Disconnect behavior remains failed; this does not implement durable disconnected-client recovery or unsolicited background delivery. Existing context-oldest reply selection is unchanged. A lost processing callback can still retain a waiter until cancellation/shutdown; review this lifecycle boundary explicitly.

Production acceptance is pending: three consecutive caller-observed replies, including one deliberately past the observation deadline. Unit/subsystem tests are not that live proof.

[RuniThomsen]

Superseded by upstream work in #96983; closing.