Fix pyright errors in test_client_credentials.py

Add explicit None checks before accessing client_info attributes
to satisfy pyright type narrowing requirements.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: pcarleton

tests/client/auth/extensions/test_client_credentials.py

@@ -208,6 +208,7 @@ async def test_init_with_scopes(self, mock_storage: MockTokenStorage):
         )
 
         await provider._initialize()
+        assert provider.context.client_info is not None
         assert provider.context.client_info.scope == "read write"
 
     @pytest.mark.anyio
@@ -222,6 +223,7 @@ async def test_init_with_client_secret_post(self, mock_storage: MockTokenStorage
         )
 
         await provider._initialize()
+        assert provider.context.client_info is not None
         assert provider.context.client_info.token_endpoint_auth_method == "client_secret_post"
 
     @pytest.mark.anyio