test : added pytest unit tests for app/ml/security.py

[tmdeveloper007]

Closes #1863.

Summary of What Has Been Done:
Added pytest tests for app/ml/security.py covering SafeUnpickler class restrictions (allowed/forbidden modules), HMAC signature computation and verification functions, and the safe_pickle_load wrapper. All 13 tests pass.

Changes Made:

• tests/test_ml_security.py (new file): 152 lines covering SafeUnpickler, compute_signature, verify_signature, write_signature, safe_pickle_load

Impact it Made:

• Guards ML model loading against pickle RCE (CWE-502)
• Increases Python test coverage for security-critical module
• Verified: python3 -m pytest tests/test_ml_security.py -v --noconftest (13 passed)

Note: Please assign this PR to the tmdeveloper007 account.

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