Fix add_object/add_event/add_todo for async clients (issue #631)

When using AsyncDAVClient, CalendarObjectResource.save() returns a
coroutine. Collection.add_object() was calling o.save() and then
immediately accessing o.url, crashing with AttributeError because
o was a coroutine, not the saved object.

Fix by routing through _async_add_object_finish() when is_async_client,
which awaits save() before checking o.url and calling _handle_reverse_relations().

Fixes #631

Co-Authored-By: Claude <noreply@anthropic.com>

Author: tobixen

caldav/collection.py

@@ -840,6 +840,8 @@ def add_object(
             ),
             parent=self,
         )
+        if self.is_async_client:
+            return self._async_add_object_finish(o, no_overwrite=no_overwrite, no_create=no_create)
         o = o.save(no_overwrite=no_overwrite, no_create=no_create)
         ## TODO: Saving nothing is currently giving an object with None as URL.
         ## This should probably be changed in some future version to raise an error
@@ -848,6 +850,13 @@ def add_object(
             o._handle_reverse_relations(fix=True)
         return o
 
+    async def _async_add_object_finish(self, o, no_overwrite=False, no_create=False):
+        """Async helper for add_object(): awaits save() then handles reverse relations."""
+        o = await o.save(no_overwrite=no_overwrite, no_create=no_create)
+        if o.url is not None:
+            o._handle_reverse_relations(fix=True)
+        return o
+
     def add_event(self, *largs, **kwargs) -> "Event":
         """
         Add an event to the calendar.