feat(contextMenu): migrate core extensions to new context menu API

Migrates core extensions from monkey-patching to the new context menu extension API, demonstrating the migration path for extension developers.

**Migrated extensions:**
- `groupOptions.ts`: Group management context menu items
- `nodeTemplates.ts`: Template save/load context menu items

**Legacy compatibility test:**
- `groupNode.ts`: Kept with monkey-patching as compatibility test case
- Includes documentation explaining the old vs new approach

**Benefits:**
- Type-safe menu item registration
- No prototype pollution
- Easier to test and maintain
- Clear separation of concerns

Depends on PR #5977 (context menu extension API)

Author: Myestery

src/extensions/core/groupNode.ts

@@ -1630,6 +1630,18 @@ export class GroupNodeHandler {
   }
 }
 
+/**
+ * LEGACY COMPATIBILITY TEST CASE
+ *
+ * This extension uses the old monkey-patching approach to modifying context menus.
+ * It is kept as-is to test backward compatibility with the new context menu API.
+ *
+ * New extensions should use the new ComfyExtension interface methods:
+ * - getCanvasMenuItems(canvas): IContextMenuValue[]
+ * - getNodeMenuItems(node): IContextMenuValue[]
+ *
+ * See nodeTemplates.ts and groupOptions.ts for examples of the new API.
+ */
 function addConvertToGroupOptions() {
   // @ts-expect-error fixme ts strict error
   function addConvertOption(options, index) {