fix(models): serialize multimodal function response display names

[MukundaKatta]

Summary

• serialize nested function_response payloads with alias-aware camelCase keys
• preserve displayName inside multimodal function-response parts so $ref names can match the uploaded media part
• add a focused regression test for the generated wire shape

Testing

• python3 -m py_compile google/genai/models.py google/genai/tests/types/test_types.py
• python3 -m pytest google/genai/tests/types/test_types.py -k function_response_display_name