[PLUGINS] Add plugin dependency preflight with OS-specific guidance

[utksh1]

Problem

SecuScan needs a production-grade improvement in this area: Scanner dependency readiness..

Scope

Detect missing binaries/python packages/system dependencies, return actionable install guidance by OS, and surface it in Toolkit/Tool Config.

Acceptance Criteria

• The implementation is focused and does not introduce unrelated UI, docs, lockfile, or formatting churn.
• Security-sensitive behavior has explicit negative tests where applicable.
• Existing tests continue to pass, and new tests cover the main success and failure paths.
• Documentation or configuration examples are updated when operator behavior changes.

Verification

Tests should cover macOS/Linux guidance, missing binary states, and runnable plugin states.

Difficulty

Hard, useful issue intended for experienced contributors.

[aaniya22]

Please assign this issue to me under Gssoc'26