Changes
PR #1685 by ksylvan: Fix Gemini Role Mapping for API Compatibility
- Fix Gemini role mapping to ensure proper API compatibility by converting chat roles to Gemini's user/model format
- Map assistant role to model role per Gemini API constraints
- Map system, developer, function, and tool roles to user role for proper handling
- Default unrecognized roles to user role to preserve instruction context
- Add comprehensive unit tests to validate convertMessages role mapping logic