feat: add plugin dependency health check on startup (#1979)

[aarushlohit]

Description

Proactively checks for required CLI dependencies when the SecuScan backend starts and surfaces warnings before the user tries to run a scan. Previously, a task would fail
abruptly at runtime if a required binary (e.g., nmap, gobuster) was missing from the system PATH.

Related Issues

Closes #1979

Type of Change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• All 27 existing unit tests in testing/backend/unit/test_plugins.py pass
• Added assertions in test_plugin_list_exposes_runtime_capabilities verifying is_available mirrors the runtime runnable state
• Added new test test_plugin_schema_includes_availability validating get_plugin_schema() returns a complete availability block
• At startup, plugins with missing binaries emit a WARNING log (e.g. Plugin Nmap (nmap) is unavailable — missing dependency binaries: nmap) and a summary line is logged
  (Plugin dependency health: 50 available, 9 unavailable)

What Changed

Backend (backend/secuscan/plugins.py)

• list_plugins() now returns is_available: bool alongside the existing availability block

• load_plugins() loops through each successfully loaded plugin at startup, calls _get_missing_binaries(), logs a WARNING per plugin missing dependencies, and logs a
  summary count of available/unavailable plugins

• get_plugin_schema() now includes the full availability dict so the /api/v1/plugin/{id}/schema endpoint also exposes dependency health

Frontend

• frontend/src/api.ts — added is_available: boolean to PluginListItem; added optional availability?: PluginAvailability to PluginSchemaResponse

• frontend/src/pages/ToolConfig.tsx — INITIATE_SCAN button is disabled when any dependency binary is missing; button label changes to MISSING_DEPENDENCIES; a tooltip line
  renders below the button listing missing binaries (e.g. Missing dependency: nmap)

• frontend/src/pages/Toolkit.tsx — plugin catalog cards are marked disabled when !plugin.availability.runnable, showing in reduced opacity/grayscale with the existing
  lock overlay

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.
• My changes generate no new warnings.
• I have added tests that verify my changes.
• New and existing unit tests pass locally.

[aarushlohit]

@utksh1