fix(editor): safeguard event unregistrations against undefined values

[mrueg]

Description

This PR safeguards the event handler unregistrations in the editor features (SignsFeature and LineAuthoringFeature) against undefined values during feature deactivation / plugin unloading, and removes a dead field.

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The Issue

When features are deactivated (or when the plugin is unloaded), EditorIntegration calls deactivateFeature() on each feature. If a feature was never activated (for example, if the feature is disabled in settings), its event variables (e.g. workspaceLeafChangeEvent, fileRenameEvent, fileOpenEvent, etc.) remain undefined.

Calling this.plg.app.workspace.offref(this.workspaceLeafChangeEvent!) directly with undefined on deactivation is unsafe.

Additionally:

• SignsFeature declared and unregistered gutterContextMenuEvent, but it was never registered or used.

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The Fix

1. Safety Checks: Updated destroyEventHandlers() in both features to verify that event references exist before calling offref() on them:

   if (this.workspaceLeafChangeEvent) {
       this.plg.app.workspace.offref(this.workspaceLeafChangeEvent);
       this.workspaceLeafChangeEvent = undefined;
   }

2. Dead Code Cleanup: Removed the unused gutterContextMenuEvent declaration and unregistration from SignsFeature.

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Verification

• Verified that compiler checks (npm run tsc) pass cleanly.
• Verified that formatting and linter checks (npm run lint) pass with no warnings.