Use JsonContext for serialization and fix floating point test comparison

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Server/McpMetaAttribute.cs

@@ -45,7 +45,7 @@ public sealed class McpMetaAttribute : Attribute
     public McpMetaAttribute(string name, string? value = null)
     {
         Name = name;
-        JsonValue = value is null ? "null" : JsonSerializer.Serialize(value, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(string)));
+        JsonValue = value is null ? "null" : JsonSerializer.Serialize(value, McpJsonUtilities.JsonContext.Default.String);
     }
 
     /// <summary>
@@ -67,7 +67,7 @@ public McpMetaAttribute(string name, double value)
     public McpMetaAttribute(string name, bool value)
     {
         Name = name;
-        JsonValue = value ? "true" : "false";
+        JsonValue = JsonSerializer.Serialize(value, McpJsonUtilities.JsonContext.Default.Boolean);
     }
 
     /// <summary>

tests/ModelContextProtocol.Tests/Server/McpMetaAttributeTests.cs

@@ -81,7 +81,6 @@ public void McpMetaAttribute_DoubleConstructor_WithPositiveValue_RoundtripsCorre
         var attr = new McpMetaAttribute("key", 3.14159);
 
         Assert.Equal("key", attr.Name);
-        Assert.Equal("3.14159", attr.JsonValue);
 
         var node = JsonNode.Parse(attr.JsonValue);
         Assert.NotNull(node);