fix: AnticipatedNetworkVariable not updating previous value [MTTB-1036] (back port) (#3322)

Back-port of #3306.

This fixes the issue where the `AnticipatedNetworkVariable` previous
value was never updated on the non-authority/write permission instances
when the `OnAuthoritativeValueChanged` callback was invoked.

[MTTB-1036](https://jira.unity3d.com/browse/MTTB-1036)

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## Changelog

- Fixed: Issue where the `AnticipatedNetworkVariable` previous value was
never updated on the non-authority/write permission instances when the
`OnAuthoritativeValueChanged` callback was invoked.

## Testing and Documentation

- Includes integration test
`NetworkVariableAnticipationTests.PreviousValueIsMaintainedProperly`.
- No documentation changes or additions were necessary.

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API:
### Deprecated API
- [ ] An `[Obsolete]` attribute was added along with a `(RemovedAfter
yyyy-mm-dd)` entry.
- [ ] An [api updater] was added.
- [ ] Deprecation of the API is explained in the CHANGELOG.
- [ ] The users can understand why this API was removed and what they
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Author: NoelStephensUnity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -14,6 +14,8 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed issue where `AnticipatedNetworkVariable` previous value returned by `AnticipatedNetworkVariable.OnAuthoritativeValueChanged` is updated correctly on the non-authoritative side. (#3322)
+
 ### Changed
 
 

com.unity.netcode.gameobjects/Runtime/NetworkVariable/AnticipatedNetworkVariable.cs

@@ -387,6 +387,9 @@ public override void ReadField(FastBufferReader reader)
         public override void ReadDelta(FastBufferReader reader, bool keepDirtyDelta)
         {
             m_AuthoritativeValue.ReadDelta(reader, keepDirtyDelta);
+            // Assure that the post delta read is invoked in order to update
+            // previous value.
+            m_AuthoritativeValue.PostDeltaRead();
         }
     }
 }

com.unity.netcode.gameobjects/Tests/Runtime/NetworkVariableAnticipationTests.cs

@@ -416,5 +416,38 @@ public void WhenStaleDataArrivesToReanticipatedVariable_ItIsAppliedAndReanticipa
             Assert.AreEqual(25, otherClientComponent.ReanticipateOnAnticipationFailVariable.Value);
             Assert.AreEqual(20, otherClientComponent.ReanticipateOnAnticipationFailVariable.AuthoritativeValue);
         }
+
+        private int m_PreviousSnapValue;
+        /// <summary>
+        /// Validates the previous value is being properly updated on the non-authoritative side.
+        /// </summary>
+        [Test]
+        public void PreviousValueIsMaintainedProperly()
+        {
+            var testComponent = GetTestComponent();
+
+            testComponent.SnapOnAnticipationFailVariable.OnAuthoritativeValueChanged += OnAuthoritativeValueChanged;
+            testComponent.SnapOnAnticipationFailVariable.Anticipate(10);
+            testComponent.SetSnapValueRpc(10);
+            WaitForMessageReceivedWithTimeTravel<NetworkVariableDeltaMessage>(m_ClientNetworkManagers.ToList());
+            // Verify the previous value is 0
+            Assert.AreEqual(0, m_PreviousSnapValue);
+            testComponent.SetSnapValueRpc(20);
+
+            WaitForMessageReceivedWithTimeTravel<NetworkVariableDeltaMessage>(m_ClientNetworkManagers.ToList());
+            // Verify the previous value is 10
+            Assert.AreEqual(10, m_PreviousSnapValue);
+
+            testComponent.SetSnapValueRpc(30);
+            WaitForMessageReceivedWithTimeTravel<NetworkVariableDeltaMessage>(m_ClientNetworkManagers.ToList());
+            // Verify the previous value is 20
+            Assert.AreEqual(20, m_PreviousSnapValue);
+        }
+
+        private void OnAuthoritativeValueChanged(AnticipatedNetworkVariable<int> anticipatedValue, in int previous, in int current)
+        {
+            m_PreviousSnapValue = previous;
+        }
+
     }
 }