Clean up namespaces (#410)

No meaningful code changes, just lots of namespace shuffling to have fewer namespaces and to put things in better namespaces.

src/ModelContextProtocol/CustomizableJsonStringEnumConverter.cs

@@ -0,0 +1,100 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
+#if !NET9_0_OR_GREATER
+using System.Reflection;
+using System.Text.Json;
+#endif
+using System.Text.Json.Serialization;
+#if !NET9_0_OR_GREATER
+using ModelContextProtocol;
+#endif
+
+// NOTE:
+// This is a workaround for lack of System.Text.Json's JsonStringEnumConverter<T>
+// 9.x support for JsonStringEnumMemberNameAttribute. Once all builds use the System.Text.Json 9.x
+// version, this whole file can be removed. Note that the type is public so that external source
+// generators can use it, so removing it is a potential breaking change.
+
+namespace ModelContextProtocol
+{
+    /// <summary>
+    /// A JSON converter for enums that allows customizing the serialized string value of enum members
+    /// using the <see cref="JsonStringEnumMemberNameAttribute"/>.
+    /// </summary>
+    /// <typeparam name="TEnum">The enum type to convert.</typeparam>
+    /// <remarks>
+    /// This is a temporary workaround for lack of System.Text.Json's JsonStringEnumConverter&lt;T&gt;
+    /// 9.x support for custom enum member naming. It will be replaced by the built-in functionality
+    /// once .NET 9 is fully adopted.
+    /// </remarks>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class CustomizableJsonStringEnumConverter<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicFields)] TEnum> :
+        JsonStringEnumConverter<TEnum> where TEnum : struct, Enum
+    {
+#if !NET9_0_OR_GREATER
+        /// <summary>
+        /// Initializes a new instance of the <see cref="CustomizableJsonStringEnumConverter{TEnum}"/> class.
+        /// </summary>
+        /// <remarks>
+        /// The converter automatically detects any enum members decorated with <see cref="JsonStringEnumMemberNameAttribute"/>
+        /// and uses those values during serialization and deserialization.
+        /// </remarks>
+        public CustomizableJsonStringEnumConverter() :
+            base(namingPolicy: ResolveNamingPolicy())
+        {
+        }
+
+        private static JsonNamingPolicy? ResolveNamingPolicy()
+        {
+            var map = typeof(TEnum).GetFields(BindingFlags.Public | BindingFlags.Static)
+                .Select(f => (f.Name, AttributeName: f.GetCustomAttribute<JsonStringEnumMemberNameAttribute>()?.Name))
+                .Where(pair => pair.AttributeName != null)
+                .ToDictionary(pair => pair.Name, pair => pair.AttributeName);
+
+            return map.Count > 0 ? new EnumMemberNamingPolicy(map!) : null;
+        }
+
+        private sealed class EnumMemberNamingPolicy(Dictionary<string, string> map) : JsonNamingPolicy
+        {
+            public override string ConvertName(string name) =>
+                map.TryGetValue(name, out string? newName) ?
+                    newName :
+                    name;
+        }
+#endif
+    }
+}
+
+#if !NET9_0_OR_GREATER
+namespace System.Text.Json.Serialization
+{
+    /// <summary>
+    /// Determines the custom string value that should be used when serializing an enum member using JSON.
+    /// </summary>
+    /// <remarks>
+    /// This attribute is a temporary workaround for lack of System.Text.Json's support for custom enum member naming
+    /// in versions prior to .NET 9. It works together with <see cref="CustomizableJsonStringEnumConverter{TEnum}"/>
+    /// to provide customized string representations of enum values during JSON serialization and deserialization.
+    /// </remarks>
+    [AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
+    internal sealed class JsonStringEnumMemberNameAttribute : Attribute
+    {
+        /// <summary>
+        /// Creates new attribute instance with a specified enum member name.
+        /// </summary>
+        /// <param name="name">The name to apply to the current enum member when serialized to JSON.</param>
+        public JsonStringEnumMemberNameAttribute(string name)
+        {
+            Name = name;
+        }
+
+        /// <summary>
+        /// Gets the custom JSON name of the enum member.
+        /// </summary>
+        public string Name { get; }
+    }
+}
+#endif