test : added unit tests for PortScanner input resolution helpers

[tmdeveloper007]

Closes #1259.

Summary of What Has Been Done:
Added a new test file testing/backend/unit/test_port_scanner_helpers.py covering the two static pure helpers on PortScanner.

Changes Made:

• New file: testing/backend/unit/test_port_scanner_helpers.py
• TestResolveScanType: 16 test cases covering bare S/T/U, lowercase, dash-prefixed (-sT), bare sT, whitespace, empty/None defaults to T, and all invalid value patterns raising ValueError
• TestResolvePorts: 19 test cases covering None/empty/top100 to empty string, top1000/all to ranges, numeric passthrough, comma-separated specs, and invalid patterns raising ValueError
• Import PortScanner from backend.secuscan.scanners.port_scanner
• ASCII only, no unicode, trailing newline at end of file

Impact it Made:

• These helpers normalise user input before building nmap plugin commands. Incorrect outputs silently produce wrong scan results.
• No existing tests covered this module.

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