feat: Add gemini-3-pro-preview

[markmcd]

Also refactors the tool support check for better forward compatibility.

Fixes #3949

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Note

Adds gemini-3-pro-preview across constants and tests, and refactors Gemini tool support detection to version-based logic.

• Gemini support
  • Add gemini/gemini-3-pro-preview to CLI MODELS, GeminiModels/GEMINI_MODELS, and context window maps (LLM_CONTEXT_WINDOW_SIZES, Gemini provider get_context_window_size).
  • Include VCR cassette tests/cassettes/test_gemini_models[gemini-gemini-3-pro-preview].yaml and add the model to parametrized tests.
• Provider logic
  • Replace Gemini tool support flags with version-based detection (supports_tools when gemini-<version> >= 1.5), removing is_gemini_2/is_gemini_1.5 usage in tests.
• Tests
  • Update Google/Gemini tests to assert supports_tools without legacy flags and validate new model works via LiteLLM and native paths.

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