Fix intermediate plugin checkbox state handling

[mdwigley]

Description

This change refactors the OnPluginEnabledChanged logic to correctly handle tri‑state checkboxes when plugins include system interfaces.

A new helper method ApplyPluginState was introduced to DRY out the duplicated logic and ensure that:

• Non‑system descriptors are toggled according to the target state.
• System descriptors are restored to their invariant state rather than being blindly overridden.
• The aggregate plugin state (IsEnabled) is recalculated based on the actual descriptor states, ensuring the checkbox accurately reflects enabled, disabled, or indeterminate states.

Related Issue

• Resolves Tri-Mode Checkbox State Not Reset with System Interfaces #128

Motivation and Context

Previously, the plugin checkbox could incorrectly display as fully enabled even when some system interfaces remained disabled and immutable.

This fix ensures the tri‑mode checkbox reflects the true state of all descriptors, preventing misleading UI states and improving consistency.

How Has This Been Tested?

• Verified toggling plugins with a mix of system and non‑system interfaces.
• Confirmed that system interfaces retain their correct state after toggling.
• Checked that the plugin checkbox now correctly shows true, false, or null depending on the actual descriptor states.
• Tested multiple toggle sequences to ensure no regressions in state handling.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Asset change (adds or updates icons, templates, or other assets)
• Documentation change (adds or updates documentation)
• Plugin change (adds or updates a plugin)

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the core logic.
  • I have linked the project issue above.
• My change requires a change to the assets.
  • I have linked the asset issue above.
• My change requires a change to the documentation.
  • I have linked the documentation issue above.
• My change requires a change to a plugin.
  • I have linked the plugin issue above.