fix: CleanDiffedSceneObjects was not clearing PendingSoftSyncObjects

[NoelStephensUnity]

Fix is based on MTT-772:
NetworkSpawnManager.CleanDiffedSceneObjects was cleaning remaining references to destroyed in-scene placed objects (which is detected and stored within the PendingSoftSyncObjects dictionary), but it was never clearing the PendingSoftSyncObjects dictionary which would result in a "key value already exists" error under the following conditions:

• Scene-A is a game lobby and is loaded when connecting. Several players connect and the game is started based on . Once all players are ready they transition to the scene of choice (Scene-B).
• Scene-B is transitioned to or loaded and contains in-scene placed NetworkObjects of which some of these in-scene placed NetworkObjects are destroyed during game play in Scene-B. Then the players transition away from Scene-B to Scene-C (post game scene because the players died or the like).
• While in Scene-C, the players decide to try the level again and are transitioned into Scene-A to before transitioning to Scene-B again.
• Upon transitioning to Scene-B a "key already exists" exception will be thrown because the same in-scene NetworkObjects destroyed are being re-synchronized but the PendingSoftSyncObjects still has the same NetworkInstanceId as a key and its value would be null because

[0xFA11]

looking at the code above, shouldn't we just iterate over PendingSoftSyncObjects, call UnityEngine.Object.Destroy on them and then do PendingSoftSyncObjects.Clear()?
I think we don't need networkObjectsToDestroy at all.

I'd suggest something like this (pseudo-code):

// Clean up any in-scene objects that had been destroyed
if (PendingSoftSyncObjects.Count > 0)
{
    foreach (var pair in PendingSoftSyncObjects)
    {
        UnityEngine.Object.Destroy(pair.Value.gameObject);
    }

    // Make sure to clear this once done destroying all remaining NetworkObjects
    PendingSoftSyncObjects.Clear();
}

[TwoTenPvP]

Yeah that should work since it doesn't modify the actual dictionary but just makes the ghost Unity References null. Either approach works, this one being cleaner.

[NoelStephensUnity]

Just applied your suggested fix @MFatihMAR

[TwoTenPvP]

Great changes 👌🏻