fix(library): serialize additionalProperties schema in OpenAPI V2

Co-authored-by: baywet <7905502+baywet@users.noreply.github.com>

Author: Copilot

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -785,9 +785,15 @@ private void SerializeAsV2(
             });
 
             // additionalProperties
-            // a schema cannot be serialized in v2
             // true is the default, no need to write it out
-            if (!AdditionalPropertiesAllowed)
+            if (AdditionalProperties is not null)
+            {
+                writer.WriteOptionalObject(
+                    OpenApiConstants.AdditionalProperties,
+                    AdditionalProperties,
+                    (w, s) => s.SerializeAsV2(w));
+            }
+            else if (!AdditionalPropertiesAllowed)
             {
                 writer.WriteProperty(OpenApiConstants.AdditionalProperties, AdditionalPropertiesAllowed);
             }

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs

@@ -718,9 +718,9 @@ public async Task SerializeConstAsEnumV20()
         }
 
         [Fact]
-        public async Task SerializeAdditionalPropertiesAsV2DoesNotEmit()
+        public async Task SerializeAdditionalPropertiesAsV2WithEmptySchemaEmits()
         {
-            var expected = @"{ }";
+            var expected = @"{ ""additionalProperties"": { } }";
             // Given
             var schema = new OpenApiSchema
             {
@@ -734,6 +734,24 @@ public async Task SerializeAdditionalPropertiesAsV2DoesNotEmit()
             Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
         }
 
+        [Fact]
+        public async Task SerializeAdditionalPropertiesAsV2WithRefSchemaEmits()
+        {
+            var expected = @"{ ""type"": ""object"", ""additionalProperties"": { ""$ref"": ""#/definitions/MyModel"" } }";
+            // Given - schema with additionalProperties pointing to a ref (dictionary case)
+            var schema = new OpenApiSchema
+            {
+                Type = JsonSchemaType.Object,
+                AdditionalProperties = new OpenApiSchemaReference("MyModel", null)
+            };
+
+            // When
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi2_0);
+
+            // Then
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
+        }
+
         [Fact]
         public async Task SerializeAdditionalPropertiesAllowedAsV2DefaultDoesNotEmit()
         {