fix: update conversation manager tests for auto-generated system message (#9687)

fix: update Anthropic mock to match real SDK's sync list() signature

The real Anthropic SDK's models.list() is a regular (non-async) method
that returns an AsyncPaginator (async-iterable). The mock used async def,
causing `async for model in client.models.list()` to iterate over a
coroutine instead of the page, silently failing with 0 models synced.

tests/managers/test_provider_manager.py

@@ -880,6 +880,8 @@ async def test_server_startup_handles_api_errors_gracefully(default_user, defaul
             for item in self._items:
                 yield item
 
+    # NOTE: The real SDK's models.list() is a regular (non-async) method that
+    # returns an AsyncPaginator (which is async-iterable).
     class MockAnthropicModels:
         def list(self):
             return MockAnthropicAsyncPage(mock_anthropic_data)