fix(a2a): set final=True for error TaskStatusUpdateEvent in event_converter

When a task transitions to TaskState.failed, the resulting
TaskStatusUpdateEvent should have final=True because the error state
is terminal — no further events will follow.

The existing from_adk_event.py already sets final=True for error events
(line 154), but event_converter.py incorrectly uses final=False, which
causes consumers to wait indefinitely for additional events after a
failure.

Closes #5388

Signed-off-by: Cocoon-Break <54054995+kuishou68@users.noreply.github.com>

Author: kuishou68

src/google/adk/a2a/converters/event_converter.py

@@ -461,7 +461,7 @@ def _create_error_status_event(
               platform_time.get_time(), tz=timezone.utc
           ).isoformat(),
       ),
-      final=False,
+      final=True,
   )