Validate that GetContainingPropertyName() is null iff IsDocumentRoot()

Fixes #27342

src/EFCore.Cosmos/Infrastructure/Internal/CosmosModelValidator.cs

@@ -67,6 +67,12 @@ protected virtual void ValidateSharedContainerCompatibility(
                     entityType.DisplayName(), ownership.PrincipalEntityType.DisplayName(), container));
             }
 
+            if (entityType.GetContainingPropertyName() != null)
+            {
+                throw new InvalidOperationException(CosmosStrings.ContainerContainingPropertyConflict(
+                    entityType.DisplayName(), container, entityType.GetContainingPropertyName()));
+            }
+
             if (!containers.TryGetValue(container, out var mappedTypes))
             {
                 mappedTypes = new List<IEntityType>();

src/EFCore.Cosmos/Properties/CosmosStrings.resx

@@ -126,6 +126,9 @@
   <data name="ConnectionStringConflictingConfiguration" xml:space="preserve">
     <value>Both the connection string and CredentialToken, account key or account endpoint were specified. Specify only one set of connection details.</value>
   </data>
+  <data name="ContainerContainingPropertyConflict" xml:space="preserve">
+    <value>The entity type '{entityType}' is mapped to the container '{container}' but it is also configured as being contained in property '{property}'.</value>
+  </data>
   <data name="CosmosNotInUse" xml:space="preserve">
     <value>Cosmos-specific methods can only be used when the context is using the Cosmos provider.</value>
   </data>

test/EFCore.Cosmos.Tests/Infrastructure/CosmosModelValidatorTest.cs

@@ -290,14 +290,24 @@ public virtual void Detects_owned_type_mapped_to_a_container()
                 nameof(OrderDetails), nameof(Order), "Details"), modelBuilder);
     }
 
+    [ConditionalFact]
+    public virtual void Detects_conflicting_containing_property()
+    {
+        var modelBuilder = CreateConventionModelBuilder();
+        modelBuilder.Entity<Customer>().ToContainer("Orders");
+        modelBuilder.Entity<Order>().ToContainer("Orders").Metadata.SetContainingPropertyName("Prop");
+
+        VerifyError(CosmosStrings.ContainerContainingPropertyConflict(nameof(Order), "Orders", "Prop"), modelBuilder);
+    }
+
     [ConditionalFact]
     public virtual void Detects_missing_discriminator()
     {
         var modelBuilder = CreateConventionModelBuilder();
         modelBuilder.Entity<Customer>().ToContainer("Orders").HasNoDiscriminator();
         modelBuilder.Entity<Order>().ToContainer("Orders");
 
-        VerifyError(CosmosStrings.NoDiscriminatorProperty(typeof(Customer).Name, "Orders"), modelBuilder);
+        VerifyError(CosmosStrings.NoDiscriminatorProperty(nameof(Customer), "Orders"), modelBuilder);
     }
 
     [ConditionalFact]