test: add mutation-style regression coverage for validation

[mythri2405]

Description

This PR adds mutation-style regression coverage for critical safe-mode validation branches in testing/backend/unit/test_validation.py.
The added tests strengthen negative-path coverage for security-sensitive validation logic and help prevent regressions caused by future refactors. The focus is on exercising existing validation behavior without modifying validation semantics.

Key additions

• Added coverage for safe-mode rejection of unresolved hostnames
• Added regression tests for public IP blocking in safe mode
• Added coverage for link-local and multicast target rejection
• Added tests for multi-record hostname resolution scenarios
• Added loopback validation coverage under configuration constraints
• Added allowed-network enforcement coverage for hostname resolution paths

Related Issues

Closes #886

Type of Change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Executed the validation test suite locally:

python -m pytest testing/backend/unit/test_validation.py -v

Result:

47 passed, 1 warning

Coverage verified for:

• Safe-mode hostname validation
• Public IP rejection in safe mode
• Link-local and multicast target blocking
• Loopback restrictions when disabled by configuration
• Multi-resolution hostname validation behavior
• Allowed-network enforcement checks

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.

[utksh1]

Please revise before merge: this PR removes existing resolve_and_validate_target coverage while adding validation tests. Keep the existing webhook/target validation regression tests and add the new cases without reducing coverage.

[utksh1]

Thanks for this contribution! I've reviewed the proposal and it looks like a valid addition. However, this branch currently has Git merge conflicts with main. Could you please pull the latest changes from upstream, resolve the conflicts, and push the rebased branch so we can continue the review process?