test : added unit tests for Settings field validation edge cases

[tmdeveloper007]

Closes #1262.

Summary of What Has Been Done:
Extended the existing testing/backend/unit/test_config_settings.py with 14 new test cases covering edge cases not addressed by the original 15 tests.

Changes Made:

• Extended: testing/backend/unit/test_config_settings.py (+122 lines)
• allowed_networks: wildcard CIDR patterns (192.168.., 10...*), defaults, single value, empty list
• cors_allowed_origins: multiple origins, default localhost, empty list
• file path defaults: database_path, data_dir, reports_dir all resolve inside PROJECT_ROOT
• no-env-vars instantiation: Settings() works with zero kwargs
• sandbox settings: defaults present, env override via kwargs
• Total: 29 tests now (15 existing + 14 new), all passing

Impact it Made:

• allowed_networks wildcards are used by the egress validation system to permit or block scan targets. Incorrect wildcard parsing silently allows unintended targets.
• The original test file covered basic instantiation but not field-level edge cases.

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