test: add comprehensive unit tests for file-generator tool

Add 25 unit tests covering:
- MIME type inference for 17 file extensions (CSV, JSON, XML, code files, etc.)
- File upload functionality with Buffer conversion
- Presigned URL support and fallback to sourceUrl
- Multiple file uploads in parallel
- Result format validation with file metadata
- Error handling for upload failures

All tests pass with 100% coverage of core functionality.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: mrjasonroy