refactor(copilot): simplify ghost mark addition by removing conditional check

Remove the mark type allowance validation and apply ghost marks directly
to text nodes within the specified range.

src/plugins/copilotPlugin.ts

@@ -149,10 +149,7 @@ function addGhostMarksToTextNodes(tr: any, from: number, to: number, markType: a
     const end = Math.min(pos + node.nodeSize, to)
     if (end <= start) return true
 
-    const $pos = tr.doc.resolve(start)
+    tr.addMark(start, end, markType.create())
-    if ($pos.parent.type.allowsMarkType?.(markType)) {
-      tr.addMark(start, end, markType.create())
-    }
     return true
   })
 }