feat(models): allow labels in Gemini API generate_content

[MukundaKatta]

Summary

• allow GenerateContentConfig.labels to flow through the Gemini Developer API request mapper instead of raising a hardcoded error
• add a focused regression test for _GenerateContentConfig_to_mldev() label serialization

Testing

• python3 -m py_compile google/genai/models.py google/genai/tests/models/test_generate_content.py
• tried GOOGLE_GENAI_REPLAYS_DIRECTORY=/tmp/python-genai-replays python3 -m pytest google/genai/tests/models/test_generate_content.py -k generate_content_labels_are_serialized_for_mldev
• local pytest import was blocked because this environment is missing google-auth (ModuleNotFoundError: No module named 'google')