fix(gemini-native): resolve functionResponse.name and replay thought_signature for synthesized tool call IDs (#2814)

* fix(gemini-native): resolve functionResponse.name and thought_signature replay for synthesized tool call IDs

Two related bugs in the Gemini Native format conversion layer:

1. **functionResponse.name resolution** (422 error): When Gemini's parallel
   function calls omit the id field, cc-switch synthesizes gemini_synth_*
   IDs. These are stored in the shadow store but can be lost in long sessions,
   causing subsequent tool_result blocks to fail. Fix: pre-scan all assistant
   messages in the request body to seed the tool_name_by_id map, and add a
   last-resort fallback that scans the current content array for matching
   tool_use blocks.

2. **thought_signature replay** (400 error): The Anthropic Messages format
   strips thoughtSignature from tool_use blocks, but Gemini requires it on
   every functionCall in multi-turn tool-use exchanges. Fix: build a
   thought_signature_by_id map from shadow turns and attach thoughtSignature
   when converting tool_use back to functionCall.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* style: run cargo fmt on transform_gemini.rs

---------

Co-authored-by: Tiancrimson <tiancrimson@gmail.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Jason <farion1231@gmail.com>