fix(gemini): add support for Gemini 3 thought signatures and optimize

[HaiyiMei]

Summary

This PR adds support for Gemini 3 models' required thoughtSignature field in tool calls and optimizes the token counting implementation.

Changes

• Added _is_gemini_3_or_newer() helper function to detect Gemini 3 models
• Updated convert_to_gemini_tool_call_invoke() to inject thoughtSignature for Gemini 3 models when not provided
• Propagated model parameter through transformation functions to enable version detection
• Optimized _clean_contents_for_gemini_api() by removing dependency on google.genai.types.FunctionResponse

Details

• For Gemini 3 models, the first function call requires a thoughtSignature field
• When no signature is provided, uses a fallback value "skip_thought_signature_validator" to bypass validation
• Reference: https://ai.google.dev/gemini-api/docs/thought-signatures

Testing

• Changes maintain backward compatibility with existing Gemini models
• Token counting optimization removes unnecessary dependency

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