fix(caldav): migrate to upstream caldav v3.0.1 to fix href handling (#629)

When Nextcloud stores CalDAV objects, the server-side filename may differ
from the VTODO/VEVENT UID. The caldav fork constructed object URLs from
the UID instead of the actual <d:href> from REPORT responses, causing
list_todos to return wrong hrefs, delete_todo to silently no-op, and
update_todo to fail.

Upstream caldav v3.0.1 fixes this in _async_request_report_build_resultlist
by passing url=self.url.join(url) when constructing result objects.

Key changes:
- Replace caldav fork with upstream caldav>=3.0.1,<4.0
- Update imports to caldav.aio module
- Add _maybe_await() helper for v3's dual-mode methods that return
  either objects or coroutines depending on async context
- Add _async_object_by_uid() to work around upstream's get_object_by_uid
  not being async-aware (it iterates a coroutine synchronously)
- Adapt save_event/save_todo (no longer return tuples)
- Pass url=calendar.url.join(href) in _search_events_by_date
- Pass include_completed=True in list_todos to match previous behavior
- Add integration test for filename != UID scenario

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/client/calendar/test_calendar_operations.py

@@ -639,11 +639,10 @@ async def test_calendar_operations_error_handling(
     # Test with non-existent calendar
     fake_calendar = f"nonexistent_calendar_{uuid.uuid4().hex}"
 
-    # caldav library returns empty list for non-existent calendars, doesn't raise
-    # Testing that it doesn't crash and returns empty results
-    events = await nc_client.calendar.get_calendar_events(fake_calendar)
-    assert isinstance(events, list)
-    # Empty list is expected for non-existent calendar
-    assert len(events) == 0
+    # caldav v3 raises NotFoundError for non-existent calendars
+    from caldav.lib.error import NotFoundError
+
+    with pytest.raises(NotFoundError):
+        await nc_client.calendar.get_calendar_events(fake_calendar)
 
     logger.info("Error handling tests completed successfully")