fix(google): add thought signature to gemini 3 pro image parts

[gr2m]

Background

generateText() / streamText() doesn't pass thought signature for image parts in response messages. That breaks multi-turn chats with gemini-3-pro-image-preview

Summary

• Gemini now supports non-PNG images in assistant messages
• Add thought signature to image parts in response.messages
• spec update: LanguageModelV3File#providerMetadata (optional field)

Manual Verification

import { google } from '@ai-sdk/google';
import { generateText } from 'ai';
import 'dotenv/config';
import { presentImages } from '../lib/present-image';

async function main() {
  const step1 = await generateText({
    model: google('gemini-3-pro-image-preview'),
    prompt: 'Create an image of Los Angeles where all car infrastructure has been replaced with bike infrastructure, trains, pedestrian zones, and parks. The image should be photorealistic and vibrant.',
  });

  await presentImages(step1.files);

  const step2 = await generateText({
    model: google('gemini-3-pro-image-preview'),
    messages: [
      ...step1.response.messages,
      {
        role: 'user',
        content: 'Now create a variation of the image, but in the style of a watercolor painting.',
      },
    ]
  });

  await presentImages(step2.files);
}

main().catch(console.error);

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Related Issues

Fixes #10441
Closes #7979

[gr2m]

I verified that body.contents[0].parts[0].thoughtSignature (or body.contents[0].parts[0].thought_signature) needs to be set in the request body.

Problem is that this will need a spec change because language-model-v3-file.ts does not defined providerMetadata?: SharedV3ProviderMetadata; yet (unlike all other LanguageModelV3Content types)

but we would need to pass it via providerMetadata here

6078060/packages/google/src/google-generative-ai-language-model.ts#L268-L274

same way we pass it for function calls above

Which means we cannot backport it.

Or I'm missing something 🤔

[gr2m]

we basically ran into #7979

[gr2m]

I will start the backport pull request, but won't merge until it's absolutely necessary, since the spec change is a breaking change und might cause friction to users unaffected by the bug this fixes.

[vercel-ai-sdk]

⚠️ Backport to release-v5.0 created but has conflicts: #10508