test : add unit tests for validation module pure helper functions

[tmdeveloper007]

Summary of What Needs to be Done:
backend/secuscan/validation.py contains helper functions that are only partially tested through the existing validate_target tests. Specifically, _net_within_allowed_networks (with its nested wildcard_to_net helper) and _parse_url_hostname have no dedicated unit tests. These functions handle complex edge cases in network validation logic.

Changes that Need to be Made:

• Add testing/backend/unit/test_validation_helpers.py
• Import the real production functions from backend.secuscan.validation
• Test _parse_url_hostname: valid https URL, valid http URL, non-HTTP scheme returns None, URL with port, URL with path, invalid URL
• Test _net_within_allowed_networks: IP within CIDR allowlist, IP outside allowlist, IP within wildcard pattern (e.g. 10...*), empty allowlist returns True, multi-address network within larger CIDR
• Test wildcard_to_net edge cases: 10...* -> 10.0.0.0/8, 192.168.. -> 192.168.0.0/16, invalid patterns (non-octet values, mixed wildcards)
• Mock settings.allowed_networks for isolation using monkeypatch

Impact that it would Provide:

• Validates network allowlist enforcement logic in isolation
• Documents expected behavior of URL parsing and network matching
• Enables safe refactoring of the validation logic without breaking guardrails

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