[driftcheck] test: add integration tests for all detectors

[yunaremaia]

Summary

Add integration tests for all drift detectors in driftcheck.

Motivation

While driftcheck has unit tests for individual detectors, integration tests that run the full scan pipeline against sample repositories would improve reliability and make it easier for contributors to verify changes.

Acceptance Criteria

• Integration tests for each detector (version, count, line-ending, etc.)
• Test fixtures with known drift scenarios
• End-to-end tests for CLI commands (scan, fix, json output)
• Tests for SARIF output generation
• All tests pass in CI

Labels

good first issue, enhancement