feat: serialize const prop from 3.1 schemas as single enum for prior versions

Refs: #2335

Author: christianoMicrosoft

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -437,7 +437,10 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             });
 
             // enum
-            writer.WriteOptionalCollection(OpenApiConstants.Enum, Enum, (nodeWriter, s) => nodeWriter.WriteAny(s));
+            var enumValue = !string.IsNullOrEmpty(Const) && version != OpenApiSpecVersion.OpenApi3_1
+                ? new List<JsonNode> { JsonValue.Create(Const)! }
+                : Enum;
+            writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));
 
             // type
             SerializeTypeProperty(writer, version);
@@ -687,7 +690,10 @@ private void SerializeAsV2(
             });
 
             // enum
-            writer.WriteOptionalCollection(OpenApiConstants.Enum, Enum, (w, s) => w.WriteAny(s));
+            var enumValue = !string.IsNullOrEmpty(Const)
+                ? new List<JsonNode> { JsonValue.Create(Const)! }
+                : Enum;
+            writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));
 
             // items
             writer.WriteOptionalObject(OpenApiConstants.Items, Items, (w, s) => s.SerializeAsV2(w));

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

@@ -643,6 +643,44 @@ public async Task WriteAsItemsPropertiesDoesNotWriteNull()
             """;
             Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(actual)));
         }
+        [Fact]
+        public async Task SerializeConstAsEnumV30()
+        {
+            // Arrange
+            var schema = new OpenApiSchema
+            {
+                Type = JsonSchemaType.String,
+                Const = "foo"
+            };
+
+
+            // Act
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
+
+            // Assert
+            var v3Node = JsonNode.Parse(actual);
+            Assert.True(v3Node!["enum"] is JsonArray singleEnum && singleEnum.Count == 1 && singleEnum[0]?.ToString() == "foo");
+            Assert.False(v3Node.AsObject().ContainsKey("const"));
+        }
+
+        [Fact]
+        public async Task SerializeConstAsEnumV20()
+        {
+            // Arrange
+            var schema = new OpenApiSchema
+            {
+                Type = JsonSchemaType.String,
+                Const = "foo"
+            };
+
+            // Act
+            var actual = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi2_0);
+
+            // Assert
+            var v2Node = JsonNode.Parse(actual);
+            Assert.True(v2Node!["enum"] is JsonArray singleEnum && singleEnum.Count == 1 && singleEnum[0]?.ToString() == "foo");
+            Assert.False(v2Node.AsObject().ContainsKey("const"));
+        }
 
 
         internal class SchemaVisitor : OpenApiVisitorBase