fix: bridge unsigned Gemini 3 tool histories

[SunmiJJW]

Gemini 3 requires a thought signature when function-call history is replayed. AstrBot preserves genuine Gemini signatures, but a tool call produced by another provider has no Google-specific metadata, so switching or falling back to Gemini 3 can fail with a 4xx validation error.

Google documents skip_thought_signature_validator for the unavoidable cross-model-history case: https://ai.google.dev/gemini-api/docs/generate-content/thought-signatures

Modifications / 改动点

• Detect Gemini 3 requests while converting AstrBot conversation history.
• Preserve genuine Google thought signatures exactly as received.
• When a function-call step has no genuine signature, add the documented sentinel only to the first call in that step (including parallel calls).
• Leave Gemini 2.5 and unsigned non-Gemini histories unchanged.
• This is NOT a breaking change. / 这不是一个破坏性变更。

Verification Steps / 验证步骤

python -m pytest tests/test_gemini_source.py -q
python -m ruff check astrbot/core/provider/sources/gemini_source.py tests/test_gemini_source.py
python -m ruff format --check astrbot/core/provider/sources/gemini_source.py tests/test_gemini_source.py

Test Results / 运行结果

• 9 passed
• Ruff check passed.
• Ruff format check passed.

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Checklist / 检查清单

• 👀 My changes have been well-tested, and verification steps/results are provided above.
• 🤓 No new dependencies are introduced.
• 😮 My changes do not introduce malicious code.

Summary by Sourcery

Support reliable Gemini 3 fallback and model switching when replayed tool histories lack Google thought signatures.

Bug Fixes:

• Allow Gemini 3 to replay unsigned cross-provider function-call histories without triggering thought-signature validation errors.

Enhancements:

• Preserve genuine Google thought signatures while applying the documented fallback sentinel only to the first unsigned function call in a tool step.
• Keep Gemini 2.5 and non-Gemini conversation histories unchanged.

Tests:

• Add coverage for unsigned cross-provider tool calls, foreign signatures, genuine Gemini signatures, and unchanged Gemini 2.5 behavior.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-31T13:45:49.402166Z	821f2c7	New commits

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