fix: Objects without observers are DestroyWithOwner (#3342)

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- Ensure objects with no observers (or only current owner as observer)
are treated as if `DontDestroyWithOwner = true` on client disconnect
- Explicitly check if childObject has new client as observer on client
connect

<!-- Add short version of the JIRA ticket to the PR title (e.g. "feat:
new shiny feature [MTT-123]") -->
[MTTB-1041](https://jira.unity3d.com/browse/MTTB-1041)
Changelog for this is in #3323.

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## Changelog

- Fixed: Ensure objects with no observers are treated as if
`DontDestroyWithOwner = true` on client disconnect

## Testing and Documentation

- No tests have been added.

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Co-authored-by: Noel Stephens <noel.stephens@unity3d.com>

Author: EmandM

com.unity.netcode.gameobjects/Runtime/Connection/NetworkConnectionManager.cs

@@ -1173,16 +1173,17 @@ internal void OnClientDisconnectFromServer(ulong clientId)
                     if (ownedObject)
                     {
                         // If destroying with owner, then always despawn and destroy (or defer destroying to prefab handler)
-                        if (!ownedObject.DontDestroyWithOwner)
+                        // Handle an object with no observers other than the current disconnecting client as destroying with owner
+                        if (!ownedObject.DontDestroyWithOwner || ownedObject.Observers.Count == 0 || (ownedObject.Observers.Count == 1 && ownedObject.Observers.Contains(clientId)))
                         {
-                            if (NetworkManager.PrefabHandler.ContainsHandler(clientOwnedObjects[i].GlobalObjectIdHash))
+                            if (NetworkManager.PrefabHandler.ContainsHandler(ownedObject.GlobalObjectIdHash))
                             {
                                 if (ownedObject.IsSpawned)
                                 {
                                     // Don't destroy (prefab handler will determine this, but always notify
                                     NetworkManager.SpawnManager.DespawnObject(ownedObject, false, true);
                                 }
-                                NetworkManager.PrefabHandler.HandleNetworkPrefabDestroy(clientOwnedObjects[i]);
+                                NetworkManager.PrefabHandler.HandleNetworkPrefabDestroy(ownedObject);
                             }
                             else
                             {
@@ -1240,6 +1241,12 @@ internal void OnClientDisconnectFromServer(ulong clientId)
                                     {
                                         continue;
                                     }
+
+                                    // Skip destroy with owner objects as they will be processed by the outer loop
+                                    if (!childObject.DontDestroyWithOwner || childObject.Observers.Count == 0 || (childObject.Observers.Count == 1 && childObject.Observers.Contains(clientId)))
+                                    {
+                                        continue;
+                                    }
                                     // If the client owner disconnected, it is ok to unlock this at this point in time.
                                     if (childObject.IsOwnershipLocked)
                                     {

com.unity.netcode.gameobjects/Runtime/Spawning/NetworkSpawnManager.cs

@@ -1975,8 +1975,8 @@ internal void DistributeNetworkObjects(ulong clientId)
                             // the owned distributable parent with the owned distributable children
                             foreach (var child in children)
                             {
-                                // Ignore the parent and any child that does not have the same owner or that is already owned by the currently targeted client
-                                if (child == ownerList.Value[i] || child.OwnerClientId != ownerList.Value[i].OwnerClientId || child.OwnerClientId == clientId)
+                                // Ignore any child that does not have the same owner, that is already owned by the currently targeted client, or that doesn't have the targeted client as an observer
+                                if (child == ownerList.Value[i] || child.OwnerClientId != ownerList.Value[i].OwnerClientId || child.OwnerClientId == clientId || !child.Observers.Contains(clientId))
                                 {
                                     continue;
                                 }