fix: shorten docstring to comply with E501 line length limit

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

tests/client/auth/extensions/test_client_credentials.py

@@ -282,7 +282,7 @@ async def test_exchange_token_client_secret_post_includes_client_id(self, mock_s
 
     @pytest.mark.anyio
     async def test_exchange_token_client_secret_post_without_client_id(self, mock_storage: MockTokenStorage):
-        """Test that client_secret_post skips body credentials when client_id is None (RFC 6749 §2.3.1 requires both)."""
+        """Test client_secret_post skips body credentials when client_id is None."""
         provider = ClientCredentialsOAuthProvider(
             server_url="https://api.example.com/v1/mcp",
             storage=mock_storage,