* fix: correct CompletionRequest messages type to match OpenAI API spec
- Changed messages field type from List[str] to List[ChatCompletionMessageParam]
- This ensures proper OpenAI API compatibility where messages should be objects with role and content fields
- Fixes type inconsistency in completion request handling
* feat(tests): Add comprehensive tests for CompletionRequest model
- Add test_completion.py for litellm.types.completion module
- Test ChatCompletionMessageParam type validation
- Test tool message format compatibility
- Test function message format (deprecated)
- Test multimodal content (text + image)
- Test default empty messages list
- Test all optional parameters
- Validate OpenAI ChatCompletion API message format compatibility