fix(server): isolate push notification failures from the task lifecycle

[Linux2010]

Description

A failure in the push-notification infrastructure (a transient error while reading PushNotificationConfigStore, or a raising push_url_validator) propagated into the event-consumer's generic failure path and rewrote an otherwise successful task as TASK_STATE_FAILED, failing message/send instead of returning the task result. Root-cause analysis and a runtime reproduction are in #1313.

This adds non-breaking guards so that push delivery stays best-effort, in both handlers:

• BasePushNotificationSender.send_notification: the get_info_for_dispatch lookup now runs inside an error boundary, and a raising push_url_validator is treated as a rejected URL (fail closed) — both log via logger.exception and degrade to "notification not sent".
• DefaultRequestHandlerV2 (EventConsumer._update_task_state): the send_notification call is wrapped in try/except so a sender failure is logged and never disturbs the task state or the event stream.
• LegacyRequestHandler._send_push_notification_if_needed: the same unguarded call existed in the legacy handler (both the blocking and streaming paths) — guarded identically.

This matches the existing design intent of _dispatch_notification, which already contains all delivery failures and returns bool.

Notes for reviewers:

• Cancellation is unaffected: all guards use except Exception, and asyncio.CancelledError derives from BaseException, so cancellation still propagates.

• The best-effort contract is now documented on the PushNotificationSender ABC and BasePushNotificationSender (custom implementations may raise; the framework logs and continues).

• Follow the CONTRIBUTING Guide.

• Pull Request title follows the conventional commits specification (fix: prefix for a SemVer patch).

• Tests and linter pass.

• Appropriate docs were updated (interface/base docstrings now state the best-effort contract).

Fixes #1313 🦕

Testing

• tests/server/tasks/test_push_notification_sender.py:
  • config-store read failure is swallowed (no raise, no POST, logged);
  • a raising push_url_validator rejects the URL without propagating.
• tests/server/request_handlers/test_default_request_handler_v2.py:
  • end-to-end: with a failing push config store, message/send still returns the Task with TASK_STATE_COMPLETED and the persisted state is untouched (previously rewritten to FAILED);
  • end-to-end: a raising custom PushNotificationSender (public interface) is equally harmless — covers the consumer-level guard.
• tests/server/request_handlers/test_default_request_handler.py:
  • a raising sender does not propagate out of _send_push_notification_if_needed.
• Lint/type: ruff check --fix, ruff format, ty check all clean (the 5 unused-ignore-comment warnings pre-exist on main).
• Full suite: 2274 passed, 0 failed; coverage 93% (gate 88%), all touched files improved.

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/agent_execution/active_task.py	95.02%	95.05%	🟢 +0.03%
src/a2a/server/cluster/database_event_stream.py	92.86%	96.94%	🟢 +4.08%
src/a2a/server/request_handlers/default_request_handler.py	97.96%	97.98%	🟢 +0.02%
src/a2a/server/tasks/base_push_notification_sender.py	94.34%	95.38%	🟢 +1.04%
Total	93.10%	93.15%	🟢 +0.05%

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