fix(chats): preserve zero-valued config overrides

[MukundaKatta]

Summary

• preserve explicit chat request configs like instead of falling back to the chat default config
• apply the same fix to sync and async chat streaming paths, which used the same falsy check
• add regression tests for sync and async send_message/send_message_stream zero-temperature overrides

Testing

• python3 -m py_compile google/genai/chats.py google/genai/tests/chats/test_send_message.py
• python3 -m pytest google/genai/tests/chats/test_send_message.py -k "zero_temperature_override" (fails during collection locally: )