fix(gemini): replay thought signatures for tool calls

[seanbrar]

Summary

Preserve Gemini function-call thought signatures and call IDs in opaque continuation state, then replay them with tool results for normal, streaming, and deferred interactions. This fixes Gemini 3 tool continuations that previously failed with 400: Function call is missing a thought_signature.

Related issue

None

Test plan

• just check
  • Ruff formatting and lint passed
  • documentation lint passed
  • mypy passed
  • 474 non-API tests passed (20 API tests deselected)
• ENABLE_API_TESTS=1 uv run pytest tests/test_api.py::test_gemini3_tool_call_signature_roundtrip -q
  • Gemini 3 Flash Preview completed the signed two-turn tool loop with HTTP 200 responses
• ENABLE_API_TESTS=1 uv run pytest tests/test_api.py::test_live_tool_calls_conversation_and_reasoning_roundtrip[gemini] -q
  • Gemini 2.5 Flash Lite completed the existing tool round-trip with HTTP 200 responses
• Added focused provider contract coverage for signature extraction, continuation replay, call/result IDs, and streaming state preservation

Notes

Intended for the 2.0.0 RC3 release line. Signatures remain provider-bound and opaque: callers should keep using the untouched Pollux continuation until a pending Gemini tool exchange is complete. Portable application-authored history intentionally does not expose provider signatures.

---

• PR title follows conventional commits
• just check passes
• Tests cover the meaningful cases, not just the happy path
• Docs updated (if this changes public API or user-facing behavior)

[codecov]

Codecov Report

❌ Patch coverage is 85.00000% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pollux/providers/gemini.py	85.00%	9 Missing ⚠️

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