Fix: Return NotFoundErr instead of Unavailable when the participant does not exist in UpdateParticipant. (#3543)

* Check if Participant exists when update metadata

* Change Test cases

* type smuggle oss participant check into roomstore

* tidy

---------

Co-authored-by: Paul Wells <paulwe@gmail.com>

test/singlenode_test.go

@@ -356,9 +356,7 @@ func TestSingleNodeUpdateParticipant(t *testing.T) {
 		require.Error(t, err)
 		var twErr twirp.Error
 		require.True(t, errors.As(err, &twErr))
-		// Note: for Cloud this would return 404, currently we are not able to differentiate between
-		// non-existent participant vs server being unavailable in OSS
-		require.Equal(t, twirp.Unavailable, twErr.Code())
+		require.Equal(t, twirp.NotFound, twErr.Code())
 	})
 }