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 support for gemini-3-pro-preview, updates context windows, and refactors Gemini tool-support detection; tests and VCR cassette added.

• Gemini provider:
  • Model support: Add gemini/gemini-3-pro-preview to CLI MODELS, GeminiModels/GEMINI_MODELS, and context window maps (LLM_CONTEXT_WINDOW_SIZES, GeminiCompletion get_context_window_size).
  • Tool support detection: Replace version flags with regex-based version parsing; supports_tools now true for gemini->=1.5.
• Tests:
  • Add VCR cassette tests/cassettes/test_gemini_models[gemini-gemini-3-pro-preview].yaml.
  • Update tests to remove deprecated is_gemini_* assertions and include gemini/gemini-3-pro-preview in parametrized cases.

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