test : added unit tests for BaseScanner pure helper methods

[tmdeveloper007]

Closes #1258.

Summary of What Has Been Done:
Added a new test file testing/backend/unit/test_scanners_base.py covering the three non-abstract, pure helper methods on BaseScanner.

Changes Made:

• New file: testing/backend/unit/test_scanners_base.py
• TestUpdateProgress: 7 test cases covering boundary clamping (negative to 0, over-1 to 1.0, in-range unchanged)
• TestGetProgress: 3 test cases for initial value and accumulation across multiple update_progress calls
• TestNormalizeSeverity: 11 test cases covering all mapping keys (critical/high/medium/moderate/low/info/informational/note), unknown defaults, case insensitivity, and non-string inputs
• Import the real BaseScanner class from backend.secuscan.scanners.base
• ASCII only, no unicode, trailing newline at end of file

Impact it Made:

• These pure, deterministic methods had zero test coverage. Tests prevent regressions as scanner plugins are extended.
• normalize_severity is a common source of bugs when new plugins emit non-standard severity strings.

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