test: add unit tests for drift_checker.py (issue #2834)

[mrveiss]

Summary

autobot-slm-backend/services/drift_checker.py was added in commit 0dba8915b (#2834) with no accompanying unit tests. The service contains non-trivial logic that warrants test coverage.

File: autobot-slm-backend/services/drift_checker.py

What needs testing

• _file_checksum() — correct SHA-256 for a known file
• _collect_checksums() — skips _SKIP_DIRS, only includes _INCLUDE_EXTENSIONS, returns correct relative paths
• compute_drift() — correctly classifies files as modified, source_only, deployed_only; handles missing source/deployed dirs gracefully; returns correct total_compared count
• build_drift_report() — correct schema shape and drift_detected flag
• get_default_source_dir() / get_default_deployed_dir() — env var override respected; fallback to root when subdir missing

Test file location

autobot-slm-backend/services/drift_checker_test.py (co-located with source, per project convention)

Related

• feat: code-sync should detect deployed-vs-source file mismatches #2834 — original implementation
• bug: path traversal in GET /code-sync/drift component query param #3427 — path traversal fix (allowlist should also be tested)