feat(genai)!: migration to google-genai

[mdrxy]

WIP

Based largely on the excellent foundational work done by @emhagman in #1077

https://googleapis.github.io/python-genai/index.html

Closes #853
Closes #1212

[oDaiSuno]

Regarding the code for _parse_response_candidate in chat_models.py, I believe we should remove the validation for None values in the content field. According to the latest Gemini API recommendations, the 3pro model now enforces the inclusion of a thought signature alongside function call.

https://ai.google.dev/gemini-api/docs/thinking?authuser=1&hl=zh-cn#signatures

    # Add function call signatures to content only if there's already other content
    # This preserves backward compatibility where content is "" for
    # function-only responses
    if function_call_signatures:
        for sig_block in function_call_signatures:
            content = _append_to_content(content, sig_block)

[mdrxy]

@oDaiSuno yep, haven't rebased this on the latest changes in main