test: add comprehensive unit tests for data models (#110)

Add 70+ comprehensive unit tests for DiscoveredSkill, Fix models
(CommandFix, FileCreationFix, FileModificationFix, MultiStepFix),
Finding, and Remediation models to achieve 90% coverage target.

Tests include:
- Validation testing using pytest.mark.parametrize
- Constructor testing with valid/invalid inputs
- Factory method testing (Finding.not_applicable, skipped, error)
- Serialization testing (to_dict methods)
- Apply/preview testing for Fix models
- Edge case testing (empty values, out-of-range scores)

Target coverage improvement: 35-70% → 90% for models

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Jeremy Eder <jeremyeder@users.noreply.github.com>

Author: jeremyeder