perf(clear_events): fix Ruff PERF203 and add Py3.13 shutdown compatibility

- Move try/except outside the loop to eliminate per-iteration exception overhead (fixes Ruff PERF203)
- Keep lock scope minimal: drain parent queue under lock, release before awaiting children
- Use asyncio.gather for concurrent child queue clearing with safe exception handling
- Handle Python 3.13 asyncio.QueueShutDown in clear_events via a version-safe except clause
  (fallback to QueueEmpty on <=3.12)

No public API changes; behavior is preserved on older Python versions.

Author: meoow113

src/a2a/server/events/event_queue.py

@@ -192,7 +192,7 @@ async def clear_events(self, clear_child_queues: bool = True) -> None:
                     )
                     self.queue.task_done()
                     cleared_count += 1
-            except asyncio.QueueEmpty:
+            except (asyncio.QueueEmpty, getattr(asyncio, 'QueueShutDown', asyncio.QueueEmpty)):
                 pass
 
             if cleared_count > 0: