[TEST] Add parser and contract coverage for plugin `container_scanner`

[utksh1]

Summary

plugins/container_scanner appears in the shipped plugin catalog, but there is no direct test coverage mentioning container_scanner under testing/backend or frontend/testing.

Why this matters

Parser or metadata regressions for this plugin can ship without any fixture-based signal. That is risky for a plugin-driven product where metadata, command rendering, and parser normalization are all part of the contract.

Scope

• Add at least one plugin-specific test that references container_scanner directly.
• Exercise metadata loading and command rendering for representative inputs.
• Verify parser output is normalized into stable SecuScan findings/results.
• Keep fixtures small and deterministic so they can run in CI.

Acceptance criteria

• A backend test file references container_scanner explicitly.
• The plugin metadata loads through the current validation path.
• Parser output or command rendering is asserted with a stable fixture.
• The test passes under pytest testing/backend -q.

[anshul23102]

Interested in contributing under NSoC'26. This is a valuable issue. Happy to work on it.

/assign