[backlog] Gemini tool-history thought_signature (upstream #580/#581)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #580, #581
    - Matrix: `docs/analysis/original-gap-matrix.md`
    - Category: feature-protocol
    - Status: partial · Priority: P1 · Backlog: yes
    - Epic: Protocol
    - Parent: G3 #10

    ## Evidence (from matrix)
    Only aggregate field `ThoughtSignatures []string` in `transform/gemini/generate_content/compatibility.go` `GeminiAggregateState`; no request-side injection/preservation of tool-history `thought_signature` for official Gemini chat. Stream state can collect signatures but native bridge fix for official tool history is incomplete vs PR intent.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] Official Gemini chat multi-turn tool history preserves/injects required `thought_signature`

• Request transform + stream aggregate round-trip covered by fixtures

• Regression test for tool-call follow-up without signature rejection

• Document any models that still lack signature support

    ## Out of scope for this issue filing
    Docs-only backlog. No product code until a later implementation wave claims this issue.