Add comprehensive unit tests for agent registry

- Test agent ID parsing and validation
- Test database queries and caching behavior
- Test local agent template assembly
- Test error handling for edge cases
- Test priority ordering (local vs database agents)
- Mock all external dependencies properly
- 17 test cases covering core functionality

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <noreply@codebuff.com>

Author: jahooma