Adds logic for serializing the Pattern Properties object in a JSON schema

Author: MaggieKimani1

src/Microsoft.OpenApi/Writers/OpenApiWriterBase.cs

@@ -1,10 +1,12 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Text.Json;
+using System.Text.RegularExpressions;
 using Json.Schema;
 using Json.Schema.OpenApi;
 using Microsoft.OpenApi.Any;
@@ -536,6 +538,16 @@ public void WriteJsonSchemaWithoutReference(IOpenApiWriter writer, JsonSchema sc
             writer.WriteOptionalMap(OpenApiConstants.Properties, (IDictionary<string, JsonSchema>)schema.GetProperties(),
                 (w, key, s) => w.WriteJsonSchema(s, version));
 
+            // pattern properties
+            var patternProperties = schema?.GetPatternProperties();
+            var stringPatternProperties = patternProperties?.ToDictionary(
+                kvp => kvp.Key.ToString(),  // Convert Regex key to string
+                kvp => kvp.Value
+            );
+
+            writer.WriteOptionalMap(OpenApiConstants.PatternProperties, stringPatternProperties,
+                (w, key, s) => w.WriteJsonSchema(s, version));
+
             // additionalProperties
             if (schema.GetAdditionalPropertiesAllowed() ?? false)
             {