test: add unit tests for drift_checker.py

[mrveiss]

Closes #3428

Changes

New file: autobot-slm-backend/services/drift_checker_test.py — 38 tests across 7 test classes:

Class	Tests	Covers
TestFileChecksum	4	SHA-256 computation, empty files, large files, uniqueness
TestCollectChecksums	7	Extension filtering, excluded dirs, relative paths, unreadable files
TestComputeDrift	13	Modified/added/removed files, missing dirs, nested paths, edge cases
TestBuildDriftReport	6	Schema keys, drift flags, timestamps, path fields
TestGetDefaultDeployedDir	3	Root path, env override, component suffix
TestGetDefaultSourceDir	3	Exists/missing dir, env var
TestAllowedComponents	2	Allowlist contents, frozenset type, path traversal absent

All tests are fully offline — no filesystem reads outside tmp_path, no Redis, no network. services.git_tracker is patched at import time via stub injection.

pytest.ini: Added --import-mode=importlib to addopts so co-located tests inside packages with heavy __init__.py (like services/) collect without triggering package __init__ before conftest stubs are in place. Verified no regression on the existing 55 SLM tests (same 22 pre-existing failures, same 33 passing before and after).

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[mrveiss]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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