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.

---

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 replaying unsigned cross-provider tool histories with Gemini 3 without changing existing Gemini 2.5 or non-Gemini behavior.

Bug Fixes:

• Prevent Gemini 3 request validation failures when replaying unsigned function-call histories from other providers.
• Preserve genuine Google thought signatures while supplying the documented fallback for unsigned Gemini 3 tool steps.

Tests:

• Add coverage for cross-provider, foreign-signature, genuine-signature, and Gemini 2.5 tool-call histories.

[sourcery-ai]

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

Sourcery assessment

Approved.

---

Sourcery is free for open source - if you like our reviews please consider sharing them ✨

• X
• Mastodon
• LinkedIn
• Facebook

_{Help me be more useful! Please click 👍 or 👎 on each comment and I'll use the feedback to improve your reviews.}

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 4359c44258

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".