test: Add parser and contract coverage for subdomain_takeover plugin

[mythri2405]

Description

This PR adds contract-level test coverage for the subdomain_takeover plugin. It ensures that plugin metadata, command rendering, and parser normalization behave consistently and do not drift silently in future changes.

The tests validate:

• Plugin metadata loading through the existing validation pipeline
• Correct command rendering via PluginManager
• Parser output structure and severity classification using deterministic fixtures
• Preservation of raw scan output in parser metadata
  This improves reliability of the plugin system by enforcing strict contracts for plugin behavior.

Related Issues

Closes #512

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Run the full backend test suite:

pytest testing/backend -q

All tests pass successfully, including:

• Metadata validation tests
• PluginManager command rendering tests
• Parser contract tests with fixed input fixtures

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.
• I have made corresponding changes to the documentation(no docs changes required).
• My changes generate no new warnings(excluding known upstream pytest/pydantic warnings).