test : added unit tests for FilterPreset and SavedViewCreate Pydantic validators

[tmdeveloper007]

Closes #1225.

Summary of What Has Been Done:
Added 13 unit tests to testing/backend/unit/test_saved_views.py covering the Pydantic model validators in FilterPreset and SavedViewCreate from backend.secuscan.saved_views. The existing integration tests cover the HTTP routes but not the validator edge cases.

Changes Made:

• FilterPreset defaults: severity defaults to all, sortMode defaults to severity, scanner defaults to all
• FilterPreset.validate_sort_mode: all 4 valid modes pass; invalid mode raises ValueError
• FilterPreset.validate_severity: all 6 valid severities pass; invalid severity raises ValueError
• SavedViewCreate.strip_name: valid name passes, whitespace is trimmed, blank/whitespace-only raises ValidationError (min_length constraint fires for empty string, field_validator fires for whitespace-only)
• SavedViewCreate.validate_filter_json: valid JSON passes, malformed JSON raises ValidationError

Impact it Made:
FilterPreset and SavedViewCreate are the input boundary for the saved-views API. Validator bugs could corrupt stored filter state. Direct unit tests catch regressions in validator logic without needing a full HTTP client.

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