fix(gemini): allow thinking_config passthrough

[autodots-AHassan]

Description

Add thinking_config to the Gemini provider allowlist so language_model_params={"thinking_config": ...} survives GeminiLanguageModel.__init__ filtering and is forwarded to the Gemini API. This also updates the provider docstring and adds regression coverage for both constructor-time and runtime passthrough.

Fixes #319

Bug fix

How Has This Been Tested?

• uv run --extra openai --extra test pytest tests/inference_test.py -k gemini

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page, and I either signed the Google Individual CLA or am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes.
• I have followed Google's Python Style Guide and ran pylint over the affected code.

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