Fix regression that de-serializes ETag as null for custom ITableEntity models (Azure#16986)

Author: christothes

sdk/tables/Azure.Data.Tables/src/Extensions/DictionaryTableExtensions.cs

@@ -205,7 +205,7 @@ internal static void CastAndRemoveAnnotations(this IDictionary<string, object> e
             }
 
             // Populate the ETag if present.
-            if (entity.TryGetValue(TableConstants.PropertyNames.ETag, out var etag))
+            if (entity.TryGetValue(TableConstants.PropertyNames.EtagOdata, out var etag))
             {
                 result.ETag = new ETag((etag as string)!);
             }

sdk/tables/Azure.Data.Tables/tests/TableClientLiveTests.cs

@@ -888,6 +888,7 @@ public async Task CreatedCustomEntitiesAreRoundtrippedProprly()
                 Assert.That(entityResults[i].PartitionKey, Is.EqualTo(entitiesToCreate[i].PartitionKey), "The entities should be equivalent");
                 Assert.That(entityResults[i].RowKey, Is.EqualTo(entitiesToCreate[i].RowKey), "The entities should be equivalent");
                 Assert.That(entityResults[i].StringTypeProperty, Is.EqualTo(entitiesToCreate[i].StringTypeProperty), "The entities should be equivalent");
+                Assert.That(entityResults[i].ETag, Is.Not.EqualTo(default(ETag)), $"ETag value should not be default: {entityResults[i].ETag}");
             }
         }