fix(chats): preserve zero-valued config overrides#2302

MukundaKatta wants to merge 2 commits intogoogleapis:maingoogleapis/python-genai:mainfrom

MukundaKatta:codex/python-genai-chat-zero-tempMukundaKatta/python-genai:codex/python-genai-chat-zero-tempCopy head branch name to clipboard