test(gemini): add verification for thought signature preservation

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🎯 Changes

Gemini 3.0 models require thought signatures from function calls to be preserved and sent back in subsequent requests. Without this, the API returns a 400 validation error.

The implementation was already complete in the previous commit. This PR adds comprehensive test coverage to verify:

• Thought signatures are captured from Gemini responses into ToolCall.metadata
• Signatures are correctly sent back to the API in follow-up messages
• The full request/response cycle maintains compatibility with Gemini 3.0

// When Gemini returns a function call with thoughtSignature
{
  functionCall: {
    name: 'get_weather',
    args: { location: 'Paris' }
  },
  thoughtSignature: 'thought_abc123'
}

// It's preserved in metadata
toolCall.metadata = { thoughtSignature: 'thought_abc123' }

// And sent back in subsequent requests
{
  functionCall: { ... },
  thoughtSignature: 'thought_abc123'  // Required by Gemini 3.0
}

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

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Pull request: #1

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Pull request overview

This PR adds comprehensive test coverage for the thought signature preservation feature required by Gemini 3.0 models. The feature itself was already implemented; this PR verifies that thought signatures are correctly captured from API responses and sent back in subsequent requests to maintain API compatibility.

Changes:

• Added a new integration test that validates the complete thought signature round-trip flow
• Tests both capturing thought signatures from function call responses and sending them back in follow-up requests

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