fix(provider): serialize Gemini thinking levels correctly

[JosephTian876]

Gemini 3 thinking levels are currently assigned through a class-level fallback on types.ThinkingConfig. On the first request this can leave the Pydantic instance without a serialized thinking_level, while also mutating SDK class state for later requests. In addition, Gemini 3.7 Flash does not accept MINIMAL and should use its supported MEDIUM fallback.

Modifications / 改动点

• Pass types.ThinkingLevel directly to the ThinkingConfig constructor so every request serializes its own value.

• Keep case-insensitive validation for Gemini 3 models.

• Use MEDIUM when Gemini 3.7 receives MINIMAL or another unsupported value; retain the existing HIGH fallback for other Gemini 3 variants.

• Add regressions proving two consecutive requests both serialize HIGH and Gemini 3.7 maps MINIMAL to MEDIUM.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

# Locked project SDK: google-genai 2.20.0
pytest tests/test_gemini_source.py
9 passed

# Declared minimum SDK compatibility: google-genai 1.56.0
uv run --with google-genai==1.56.0 --no-sync \
  python -m pytest tests/test_gemini_source.py
9 passed

ruff format --check .
502 files already formatted

ruff check .
All checks passed!

No dependency or API-schema change is introduced. This PR intentionally does not add a Gemini-local retry for empty STOP responses: #7104 already handles empty model output in the shared runner, and another provider-local retry would multiply retry attempts.

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc. / Not applicable: this is a bug fix and adds no user-facing feature.
• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
• 😮 My changes do not introduce malicious code.

Summary by Sourcery

Fix Gemini thinking-level serialization and validation across supported Gemini 3 models.

Bug Fixes:

• Serialize Gemini thinking levels independently for every request and apply model-specific fallbacks, including mapping unsupported Gemini 3.7 Flash values to MEDIUM.

Tests:

• Add regression coverage for repeated HIGH-level serialization and Gemini 3.7 MINIMAL fallback behavior.

[sourcery-ai]

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

Sourcery assessment

Approved.

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