feat(models): add Gemini 3.1 Pro/Flash support

- Add model ID normalization for gemini-3.1-pro and gemini-3.1-flash
- Add explicit capability matchers for Gemini 3.1 models
- Add unit tests for Gemini 3.1 normalization and capabilities
- Update README.md with Gemini 3.1 documentation
- Fix broken documentation links in README.md
- Add new configuration options to README.md
- Suppress informational Pylance warnings in pyrightconfig.json