refactor: use OpenApiConstants for extension name

- Added UnevaluatedPropertiesExtension constant to OpenApiConstants
- Replaced hardcoded "x-jsonschema-unevaluatedProperties" strings with constant
- Updated both V3.0 and V2.0 serialization paths
- Updated PublicAPI.Unshipped.txt with new constant

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

Author: Copilot

src/Microsoft.OpenApi/Models/OpenApiConstants.cs

@@ -125,6 +125,11 @@ public static class OpenApiConstants
         /// </summary>
         public const string UnevaluatedProperties = "unevaluatedProperties";
 
+        /// <summary>
+        /// Extension: x-jsonschema-unevaluatedProperties
+        /// </summary>
+        public const string UnevaluatedPropertiesExtension = "x-jsonschema-unevaluatedProperties";
+
         /// <summary>
         /// Field: Version
         /// </summary>

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -546,14 +546,14 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
                 if (UnevaluatedPropertiesSchema is not null)
                 {
                     writer.WriteOptionalObject(
-                        "x-jsonschema-unevaluatedProperties",
+                        OpenApiConstants.UnevaluatedPropertiesExtension,
                         UnevaluatedPropertiesSchema,
                         callback);
                 }
                 // Write boolean false as extension if explicitly set to false
                 else if (!UnevaluatedProperties)
                 {
-                    writer.WritePropertyName("x-jsonschema-unevaluatedProperties");
+                    writer.WritePropertyName(OpenApiConstants.UnevaluatedPropertiesExtension);
                     writer.WriteValue(false);
                 }
             }
@@ -819,14 +819,14 @@ private void SerializeAsV2(
             if (UnevaluatedPropertiesSchema is not null)
             {
                 writer.WriteOptionalObject(
-                    "x-jsonschema-unevaluatedProperties",
+                    OpenApiConstants.UnevaluatedPropertiesExtension,
                     UnevaluatedPropertiesSchema,
                     (w, s) => s.SerializeAsV2(w));
             }
             // Write boolean false as extension if explicitly set to false
             else if (!UnevaluatedProperties)
             {
-                writer.WritePropertyName("x-jsonschema-unevaluatedProperties");
+                writer.WritePropertyName(OpenApiConstants.UnevaluatedPropertiesExtension);
                 writer.WriteValue(false);
             }
 

src/Microsoft.OpenApi/PublicAPI.Unshipped.txt

@@ -1,4 +1,5 @@
 #nullable enable
+const Microsoft.OpenApi.OpenApiConstants.UnevaluatedPropertiesExtension = "x-jsonschema-unevaluatedProperties" -> string!
 Microsoft.OpenApi.IOpenApiSchemaWithUnevaluatedProperties
 Microsoft.OpenApi.IOpenApiSchemaWithUnevaluatedProperties.UnevaluatedPropertiesSchema.get -> Microsoft.OpenApi.IOpenApiSchema?
 Microsoft.OpenApi.OpenApiSchema.UnevaluatedPropertiesSchema.get -> Microsoft.OpenApi.IOpenApiSchema?