fix(extmark): builtin completion can still affect nearby extmarks #31387

Problem:
Built-in completion can still affect nearby extmarks. #31384

Solution:
Restore extmarks when completion leader changes.

(cherry picked from commit c7ec010ade)

src/nvim/insexpand.c

@@ -1735,6 +1735,7 @@ static void ins_compl_new_leader(void)
   ins_compl_del_pum();
   ins_compl_delete();
   ins_bytes(compl_leader + get_compl_len());
+  restore_orig_extmarks();
   compl_used_match = false;
 
   if (compl_started) {