Add initial unit tests for Phase 1 of test coverage improvement:
- telemetry: test initialization, payload sanitization, early returns (14.3% → 62.9%)
- plugin: test _parse_plugin_id static method
- rag: test _to_i18n_name static method
- persistence: test serialize_model with datetime handling
Overall core coverage: 41.9% → 42.2%
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>