fix(google): Fix 400 Bad Request error with Gemini 3 thought signatures

[rabi]

Summary

Gemini 3 models require thought_signature in tool calls and responses, and thinking content must be echoed back in the conversation loop. Changes: - Replace thought_signature field with generic metadata field on ToolRequest/ToolResponse - Add ProviderMetadata type alias for provider-specific data - Google provider stores thoughtSignature in metadata - Helper functions for Google-specific metadata extraction - Preserve thinking content in conversation when tool calls are processed - Handle signature inheritance for multi-part responses - Text-only responses treated as final answer (not thinking)

Type of Change

• [ ] Feature
• [x] Bug fix
• [x] Refactor / Code quality
• [ ] Performance improvement
• [ ] Documentation
• [x] Tests
• [ ] Security fix
• [ ] Build / Release
• [ ] Other (specify below)

AI Assistance

• [x] This PR was created or reviewed with AI assistance

Testing

Unit tests and manual testing

Related Issues

Relates to #5792 Relates to #6091

[DOsinga]

thanks @rabi for diving on this. this is a good start, but I am not sure it does everything yet. the spec says that gemini can reply with thinking messages and that we then have to keep the conversation going.

other things to note: shouldn't we just always have an optional thought signature on toolcall/response messages? the testing seems rather convoluted, I'd rather do one big round-trip thing where we construct a conversation in our native format and, convert it to google, have some asserts and then convert it back and then verify that it is the same. that would be then a parttern we can easily copy into other format checkers (since they need to be cross provider compatible)

let me know if you want to continue working on this, I do really appreciate the effort and gemini has been annoying in how they change their api without telling us

[DOsinga]

here's the related bug report: https://github.com/block/goose/issues/6091

[rabi]

let me know if you want to continue working on this, I do really appreciate the effort and gemini has been annoying in how they change their api without telling us

Thanks @DOsinga for reviewing. I think I've addressed your comments in the last push. Let me know.