Fix JsonSchemaExporter handling of extension data with global Disallow

src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs

@@ -219,8 +219,7 @@ private static JsonSchema MapJsonSchemaCore(
                     List<string>? required = null;
                     JsonSchema? additionalProperties = null;
 
-                    JsonUnmappedMemberHandling effectiveUnmappedMemberHandling = typeInfo.UnmappedMemberHandling ?? typeInfo.Options.UnmappedMemberHandling;
-                    if (effectiveUnmappedMemberHandling is JsonUnmappedMemberHandling.Disallow)
+                    if (typeInfo.EffectiveUnmappedMemberHandling is JsonUnmappedMemberHandling.Disallow)
                     {
                         additionalProperties = JsonSchema.CreateFalseSchema();
                     }

src/libraries/System.Text.Json/tests/Common/JsonSchemaExporterTests.cs

@@ -214,6 +214,34 @@ public void TypeWithDisallowUnmappedMembers_AdditionalPropertiesFailValidation()
             AssertDoesNotMatchSchema(schema, jsonWithUnmappedProperties);
         }
 
+        [Theory]
+        [InlineData(JsonUnmappedMemberHandling.Skip)]
+        [InlineData(JsonUnmappedMemberHandling.Disallow)]
+        public void TypeWithExtensionData_GlobalUnmappedMemberHandling_AllowsAdditionalProperties(JsonUnmappedMemberHandling unmappedMemberHandling)
+        {
+            const string Json = """{"Name":"name","x":42}""";
+            JsonSerializerOptions options = new(Serializer.DefaultOptions) { UnmappedMemberHandling = unmappedMemberHandling };
+            JsonTypeInfo<PocoWithExtensionDataProperty> typeInfo = Serializer.GetTypeInfo<PocoWithExtensionDataProperty>(options);
+
+            PocoWithExtensionDataProperty? value = JsonSerializer.Deserialize(Json, typeInfo);
+            Assert.NotNull(value);
+            Assert.NotNull(value.ExtensionData);
+            Assert.Equal(42, Assert.IsType<JsonElement>(value.ExtensionData["x"]).GetInt32());
+
+            string serialized = JsonSerializer.Serialize(value, typeInfo);
+            JsonTestHelper.AssertJsonEqual(Json, serialized);
+
+            foreach (JsonNode schema in new[]
+            {
+                options.GetJsonSchemaAsNode(typeof(PocoWithExtensionDataProperty)),
+                typeInfo.GetJsonSchemaAsNode()
+            })
+            {
+                Assert.False(schema.AsObject().ContainsKey("additionalProperties"));
+                AssertDocumentMatchesSchema(schema, JsonNode.Parse(serialized));
+            }
+        }
+
         [Fact]
         public void GetJsonSchemaAsNode_NullInputs_ThrowsArgumentNullException()
         {