fix(llm): preserve compatible tool signatures

[tiaoc]

Issue for this PR

Closes #8321

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

OpenAI-compatible providers may attach provider-owned continuation data to streamed tool calls through tool_calls[].extra_content.google.thought_signature. The OpenAI Chat parser previously discarded this field, so canonical tool-call history lost the signature and follow-up requests omitted it.

This change:

• parses extra_content.google.thought_signature from streamed OpenAI Chat tool-call deltas,
• stores it as canonical tool-call provider metadata,
• replays it on subsequent assistant tool calls as extra_content.google.thought_signature.

This keeps the provider extension intact across tool-call continuation for providers that require it with the function call.

How did you verify your code works?

• Updated the OpenAI Chat stream test to include extra_content.google.thought_signature and verify that it survives tool-call assembly.
• Added a request-lowering test that verifies historical tool calls replay the signature.

Screenshots / recordings

Not applicable; this is a protocol-level change with no UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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