Add unit test coverage for cross-step detection logic

[Hozyne-OpenBak]

Issue 1: Add Unit Test Coverage for Cross-Step Detection Logic

Labels: enhancement, testing, good-first-issue

Problem Statement

The cross-step detection logic (READ → TRANSFORM → SEND) lacks comprehensive unit test coverage. This makes it harder to validate edge cases and catch regressions when modifying the detection engine.

Current integration tests exist, but unit-level coverage for individual policy evaluation steps is incomplete.

Proposed Approach

1. Add unit tests in backend/src/policies/__tests__/crossStepDetection.test.js
2. Cover core scenarios:
   • Valid exfiltration chains (should block)
   • Benign multi-step sequences (should allow)
   • Partial chains that don't complete (should allow with lower risk)
   • Session timeout edge cases
   • Empty/null session history
   • Concurrent session tracking
3. Mock session storage to isolate policy logic
4. Use Jest's parameterized tests for input variations

Acceptance Criteria

• Unit tests cover at least 85% of cross-step detection code paths
• All major edge cases have explicit test coverage
• Tests run in under 2 seconds
• No integration test dependencies (pure unit tests)
• Tests follow existing Jest conventions

Estimated Complexity

4-6 hours

Files to Modify

• backend/src/policies/__tests__/crossStepDetection.test.js (create)
• Possibly: backend/src/policies/crossStepDetection.js (refactor for testability if needed)