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

AI Assistance

• 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: #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.

[cgwalters]

FWIW I did a skim through this PR and it looks sane, though I am not an expert in some of the core tool calling APIs. Having a generic extensible metadata looks like the right thing to do for sure.

It also does fix the problem for me.

[DOsinga]

I like, looks clean - if you could remove the more obvious comments, I'd say it is good to go

[alexhancock]

Yeah I concur with @DOsinga let's remove the comments which aren't adding much to what the code says, then get this in

[rabi]

I like, looks clean - if you could remove the more obvious comments, I'd say it is good to go

Done!

[DOsinga]

Nice. thanks and sorry for taking so long. Did you check that this is backwards compatible?

[DOsinga]

does this break compatibility with sessions where we've already written out the thought_signature?

[rabi]

No, it does not. I tested resuming session with builds before and after the PR and it worked fine. I also tested with older gemini models like gemini-2.0-flash and did not see any issues. Also, I notice that the thought_signature is a recent addition cfdf015.

[DOsinga]

excellent, thank you so much!