Fix IsJsonCollectionColumn for nested complex collections inside complex references

was already applied to the owned entity branch. Without this, a nested
ComplexCollection inside a ComplexProperty (reference) would falsely
classify the root JSON column as a collection, producing '[]' instead
of '{}'. Added regression test.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: roji

src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs

@@ -1288,7 +1288,8 @@ private static bool IsJsonCollectionColumn(JsonColumn jsonColumn)
         => jsonColumn.Table.ComplexTypeMappings.Any(
                m => m.TypeBase is IComplexType ct
                    && ct.GetContainerColumnName() == jsonColumn.Name
-                   && ct.ComplexProperty.IsCollection)
+                   && ct.ComplexProperty.IsCollection
+                   && !ct.ComplexProperty.DeclaringType.IsMappedToJson())
            || jsonColumn.Table.EntityTypeMappings.Any(
                m => m.TypeBase is IEntityType et
                    && et.GetContainerColumnName() == jsonColumn.Name

test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs

@@ -10053,6 +10053,54 @@ public virtual void Add_complex_collection_mapped_to_json_uses_empty_array_as_de
                 Assert.IsType<DropColumnOperation>(downOps[0]);
             });
 
+    [ConditionalFact]
+    public virtual void Add_complex_reference_with_nested_collection_mapped_to_json_uses_empty_object_as_default_value()
+        => Execute(
+            _ => { },
+            source =>
+            {
+                source.Entity(
+                    "Entity", e =>
+                    {
+                        e.Property<int>("Id").ValueGeneratedOnAdd();
+                        e.HasKey("Id");
+                    });
+            },
+            target =>
+            {
+                target.Entity(
+                    "Entity", e =>
+                    {
+                        e.Property<int>("Id").ValueGeneratedOnAdd();
+                        e.HasKey("Id");
+
+                        e.ComplexProperty<MyJsonComplex>(
+                            "ComplexReference", cp =>
+                            {
+                                cp.IsRequired();
+                                cp.ToJson("json_reference");
+                                cp.Property(x => x.Value);
+                                cp.Property(x => x.Date);
+                                cp.ComplexCollection(
+                                    x => x.NestedCollection, nc => { });
+                            });
+                    });
+            },
+            upOps =>
+            {
+                Assert.Equal(1, upOps.Count);
+
+                var operation = Assert.IsType<AddColumnOperation>(upOps[0]);
+                Assert.Equal("Entity", operation.Table);
+                Assert.Equal("json_reference", operation.Name);
+                Assert.Equal("{}", operation.DefaultValue);
+            },
+            downOps =>
+            {
+                Assert.Equal(1, downOps.Count);
+                Assert.IsType<DropColumnOperation>(downOps[0]);
+            });
+
 #pragma warning disable EF8001 // Owned JSON entities are obsolete
     [ConditionalFact]
     public virtual void Add_owned_collection_mapped_to_json_has_nullable_column()