Make enums serialize as strings if using the reflection-based serializer. (#473)

* Make enums serialize as strings if using the reflection-based serializer.

* Remove unnecessary partial

* Use an explicit reflection-based converter for enums.

* Update comments

* Make the reflection-based enum converter public.

tests/ModelContextProtocol.Tests/McpJsonUtilitiesTests.cs

@@ -1,4 +1,6 @@
 using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Text.Json.Serialization.Metadata;
 
 namespace ModelContextProtocol.Tests;
 
@@ -22,4 +24,27 @@ public static class McpJsonUtilitiesTests
 
         Assert.Equal(JsonSerializer.IsReflectionEnabledByDefault, options.TryGetTypeInfo(anonType, out _));
     }
+
+    [Fact]
+    public static void DefaultOptions_UnknownEnumHandling()
+    {
+        var options = McpJsonUtilities.DefaultOptions;
+
+        if (JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            Assert.Equal("\"A\"", JsonSerializer.Serialize(EnumWithoutAnnotation.A, options));
+            Assert.Equal("\"A\"", JsonSerializer.Serialize(EnumWithAnnotation.A, options));
+        }
+        else
+        {
+            options = new(options) { TypeInfoResolver = new DefaultJsonTypeInfoResolver() };
+            Assert.Equal("1", JsonSerializer.Serialize(EnumWithoutAnnotation.A, options));
+            Assert.Equal("\"A\"", JsonSerializer.Serialize(EnumWithAnnotation.A, options));
+        }
+    }
+
+    public enum EnumWithoutAnnotation { A = 1, B = 2, C = 3 }
+
+    [JsonConverter(typeof(JsonStringEnumConverter<EnumWithAnnotation>))]
+    public enum EnumWithAnnotation { A = 1, B = 2, C = 3 }
 }