fix: require voice connection to restore queue

tests/test_music_player.py

@@ -289,6 +289,13 @@ class MusicPlayerQueueTest(unittest.IsolatedAsyncioTestCase):
             await asyncio.sleep(0)
             self.assertEqual([track.id for track in player.state(GUILD).queue], ["restored"])
 
+    async def test_restore_requires_voice_connection(self):
+        player, _ = build_player()
+        player.voice_client = lambda guild_id: None  # type: ignore[method-assign]
+
+        with self.assertRaisesRegex(ValueError, "not connected"):
+            await player.restore_queue(GUILD)
+
 
 class LoopModeTest(unittest.TestCase):
     def test_cycle_order(self):