feat(gemini): add thinking_config to handled_keys support Gemini 2.0

[Eatosin]

Description

Added thinking_config to the handled_keys list in langextract/providers/gemini.py. This enables support for Gemini 2.0 Flash Thinking and other reasoning models that require this parameter. Previously, passing thinking_config raised a TypeError.

Fixes #319

Feature

How Has This Been Tested?

I ran a local verification script initializing the GeminiLanguageModel with the new parameter.

python -c "from langextract.providers.gemini import GeminiLanguageModel; GeminiLanguageModel(model_id='gemini-2.0-flash-thinking-exp', thinking_config={'include_thoughts': True})"

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.