fix(go/plugins/googlegenai): include thoughtSignature in part metadata

[hugoaguirre]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Summary of Changes

Hello @hugoaguirre, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the handling of thoughtSignature metadata within the Google GenAI plugin. It ensures that thoughtSignature values are correctly captured and propagated during conversions between the Google GenAI API's genai.Part and the internal ai.Part structures, improving data fidelity and consistency. Additionally, it enhances error handling for unrecognized part types.

Highlights

• ThoughtSignature Preservation (Ingress): The translateCandidate function now extracts ThoughtSignature from genai.Part objects and stores it in the ai.Part's metadata when converting from the Google GenAI format to the internal ai format.
• ThoughtSignature Propagation (Egress): The toGeminiPart function has been updated to consistently retrieve ThoughtSignature from the ai.Part's metadata and assign it to the genai.Part.ThoughtSignature field, ensuring this metadata is correctly propagated when converting to the Google GenAI format.
• Error Handling Improvement: The toGeminiPart function now returns a structured error for unknown part types instead of causing a panic, improving robustness.

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[gemini-code-assist]

Code Review

This pull request refactors the toGeminiPart function to better handle thoughtSignature and improves robustness by replacing a panic with a returned error. It also updates translateCandidate to include the thoughtSignature in the metadata of response parts. The refactoring is a positive change, enhancing code clarity and maintainability. However, I've identified a critical issue in translateCandidate where the thoughtSignature is unintentionally dropped for thought parts, which I've detailed in a specific comment.

[hugoaguirre]

Added signature to the Thought parts. Gemini documentation does not state otherwise.
Since we are manipulating Gemini parts internally, we should preserve Thought parts as they are.