Use Symbol for CollaborationMode slot name

Changes the CollaborationMode SlotFill to use a Symbol instead of a
string for the slot name. This makes the slot truly private by ensuring
only code with a reference to the exported Fill/Slot components can
access it.

String-based slot names can be accessed by anyone who knows the name.
Using Symbol() prevents unauthorized usage and follows the pattern
established in PR WordPress#67238.

Author: shekharnwagh

packages/editor/src/components/collaboration-mode/index.tsx

@@ -3,7 +3,7 @@
  */
 import { createSlotFill } from '@wordpress/components';
 
-const { Fill, Slot } = createSlotFill( 'CollaborationMode' );
+const { Fill, Slot } = createSlotFill( Symbol( 'CollaborationMode' ) );
 
 /**
  * Renders the SlotFill for collaboration mode.