[MM-63774] Allow users to leave private channels when there is only 1 member (#30746)

* allow last user to leave private channel
---------
Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/channel_test.go

@@ -897,6 +897,17 @@ func TestLeaveChannel(t *testing.T) {
 		require.Nil(t, appErr)
 		require.Len(t, threads.Threads, 1)
 	})
+
+	t.Run("can leave private channel as last member", func(t *testing.T) {
+		channel := th.createChannel(th.Context, th.BasicTeam, model.ChannelTypePrivate)
+
+		count, appErr := th.App.GetChannelMemberCount(th.Context, th.BasicChannel.Id)
+		require.Nil(t, appErr, "It should remove channel membership")
+		require.Equal(t, int64(1), count)
+
+		appErr = th.App.LeaveChannel(th.Context, channel.Id, th.BasicUser.Id)
+		require.Nil(t, appErr)
+	})
 }
 
 func TestLeaveLastChannel(t *testing.T) {