Fix stdio encoding issue: Enforce explicit UTF-8 for correct Unicode handling (#73)

* fix: Enforce UTF-8 encoding for stdio transport

This change replaces the default system encoding with an explicit UTF8Encoding (without BOM)
for both client and server transports. This ensures proper handling of Unicode characters,
including Chinese characters and emoji.

- Use UTF8Encoding explicitly for StreamReader and StreamWriter.
- Add tests for Chinese characters ("上下文伺服器") and emoji (🔍🚀👍) to confirm the fix.

Fixes #35.

* Remove relaxed JSON encoder

* Address PR feedback

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol/Utils/Json/McpJsonUtilities.cs

@@ -1,7 +1,6 @@
 using ModelContextProtocol.Protocol.Messages;
 using ModelContextProtocol.Protocol.Types;
 using System.Diagnostics.CodeAnalysis;
-using System.Text.Encodings.Web;
 using System.Text.Json;
 using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
@@ -26,11 +25,6 @@ public static partial class McpJsonUtilities
     /// <item>Enables string-based enum serialization as implemented by <see cref="JsonStringEnumConverter"/>.</item>
     /// <item>Enables <see cref="JsonIgnoreCondition.WhenWritingNull"/> as the default ignore condition for properties.</item>
     /// <item>Enables <see cref="JsonNumberHandling.AllowReadingFromString"/> as the default number handling for number types.</item>
-    /// <item>
-    /// Enables <see cref="JavaScriptEncoder.UnsafeRelaxedJsonEscaping"/> when escaping JSON strings.
-    /// Consuming applications must ensure that JSON outputs are adequately escaped before embedding in other document formats,
-    /// such as HTML and XML.
-    /// </item>
     /// </list>
     /// </para>
     /// </remarks>
@@ -58,15 +52,13 @@ public static partial class McpJsonUtilities
                 Converters = { new JsonStringEnumConverter() },
                 DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
                 NumberHandling = JsonNumberHandling.AllowReadingFromString,
-                Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
             };
         }
         else
         {
+            // Keep in sync with any additional settings above beyond what's in JsonContext below.
             options = new(JsonContext.Default.Options)
             {
-                // Compile-time encoder setting not yet available
-                Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
             };
         }
 
@@ -77,7 +69,8 @@ public static partial class McpJsonUtilities
     internal static JsonTypeInfo<T> GetTypeInfo<T>(this JsonSerializerOptions options) =>
         (JsonTypeInfo<T>)options.GetTypeInfo(typeof(T));
 
-    internal static JsonElement DefaultMcpToolSchema = ParseJsonElement("{\"type\":\"object\"}"u8);
+    internal static JsonElement DefaultMcpToolSchema { get; } = ParseJsonElement("""{"type":"object"}"""u8);
+
     internal static bool IsValidMcpToolSchema(JsonElement element)
     {
         if (element.ValueKind is not JsonValueKind.Object)
@@ -129,5 +122,4 @@ public static partial class McpJsonUtilities
         Utf8JsonReader reader = new(utf8Json);
         return JsonElement.ParseValue(ref reader);
     }
-
 }