AI Bridge reserializes OpenAI chat-completions requests before sending
them upstream. For Gemini OpenAI-compatible routes, that OpenAI
typed-parameter round trip drops
`tool_calls[].extra_content.google.thought_signature`, so Google rejects
tool-result continuations with `Function call is missing a
thought_signature`.

This PR:
- patches the AI Bridge upstream serialization boundary for Gemini
OpenAI-compatible chat completions
- shares the Gemini thought-signature patching helpers with chatd's
OpenAI-compatible transport patch to keep behavior consistent
- treats direct Google OpenAI-compatible upstream endpoints as
Gemini-scoped even when the request model is an alias
- adds the Google fallback thought signature to every assistant tool
call in the active turn, including parallel tool calls
- covers the regression that `extra_content` is dropped before the
upstream body is patched

> Mux updated this PR description on behalf of Mike.

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Co-authored-by: Susana Cardoso Ferreira <susana@coder.com>
(cherry picked from commit c349ea6)