test : added unit tests for finding_intelligence _parse_timestamp and _stable_id helpers

[tmdeveloper007]

Closes #1249.

Summary of What Has Been Done:
Added unit tests for _parse_timestamp and _stable_id in backend/secuscan/finding_intelligence.py, two foundational pure helpers used throughout the finding normalization pipeline. Tests cover datetime conversion, timezone handling, invalid inputs, and stable ID determinism.

Changes Made:

• Created testing/backend/unit/test_finding_intelligence_timestamp.py with 19 test cases
• _parse_timestamp: 9 tests (aware/naive datetime, ISO strings with/without Z, invalid strings, empty, None, microseconds, timezone conversion)
• _stable_id: 10 tests (prefix, determinism, different inputs, None/empty/whitespace parts, case normalization, length)

Impact it Made:

• Guards the foundation of the finding normalization pipeline
• Ensures stable IDs are deterministic across runs
• Increases overall test coverage of the finding_intelligence module

Note: This task is being handled by tmdeveloper007 -- please assign to that account when picking it up.