fix(a2a): honor configured reply timeout in watchdog

[rtx9jckz2g-prog]

Summary

• derive the A2A orphan watchdog deadline from A2A_REPLY_TIMEOUT
• keep the existing five-minute minimum while allowing configured long-running replies
• add one watchdog interval of grace so the watchdog cannot race the request waiter at the same deadline
• add regression coverage for long reply timeouts and the existing minimum

Problem

A2A_REPLY_TIMEOUT can be configured above 300 seconds, but the orphan watchdog always failed pending tasks after a fixed 300 seconds. A task that legitimately needed longer than five minutes could therefore be marked failed before the configured reply deadline and later still produce a response.

Related work

• fix(a2a): reply watchdog honors the route configured timeout #90158 covers per-route timeouts, but its default/root route still falls back to the fixed orphan floor rather than A2A_REPLY_TIMEOUT.
• fix(a2a): make reply deadline config-native #91686 is the broader config-native alternative. This PR intentionally keeps the existing configuration surface and fixes only the observed watchdog/reply-deadline contradiction with a small tested diff.

Verification

• uv run --with pytest pytest -q tests/plugins/test_a2a_plugin.py tests/plugins/test_a2a_phase23.py tests/plugins/test_a2a_schema_registration.py tests/tools/test_a2a_reason_roundtrip.py
• uv run python -m py_compile plugins/platforms/a2a/adapter.py tests/plugins/test_a2a_plugin.py
• ruff check plugins/platforms/a2a/adapter.py tests/plugins/test_a2a_plugin.py

[alt-glitch]

This was generated by AI during triage.

Related: #90158 already repairs the same watchdog-versus-configured-timeout failure using each served route's timeout. #91686 is the config-native companion approach; maintainers should choose one timeout authority.

[teknium1]

The max(floor, A2A_REPLY_TIMEOUT) half of this is landing via #110219 (credited in the body), together with live-waiter protection so the store cannot diverge from the wire. Will close with credit once that merges.

[teknium1]

Thanks — this landed on main via #110219 (138e426f62f3) with your commits/credit preserved. #107840: the orphan-watchdog half is covered; its core agent/ changes were not taken.
Closing in favour of the merged salvage.