feat(mistral): add FIM support for codestral models

- Add supportsFim(), completeFim(), and streamFim() methods to MistralHandler
- FIM support is only enabled for codestral models (codestral-latest, codestral-2405, etc.)
- Uses Mistral's v1/fim/completions endpoint with proper base URL routing
- Supports both streaming and non-streaming FIM completions
- Add comprehensive test coverage with 10 test cases
- Update continuedev Mistral LLM to accurately check model support
- Implementation follows the same pattern as KiloCode OpenRouter handler

Closes: FIM functionality now works for Mistral's codestral models

Author: markijbema