Use PlatformDetection class for .NET Framework detection

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

Author: Copilot

tests/ModelContextProtocol.Tests/McpProtocolExceptionDataTests.cs

@@ -68,15 +68,6 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
         });
     }
 
-    // On .NET Framework, Exception.Data requires values to be serializable with [Serializable].
-    // JsonElement is not marked as serializable, so these tests are skipped on .NET Framework.
-    private static bool IsNetFramework =>
-#if NET
-        false;
-#else
-        true;
-#endif
-
     [Fact]
     public async Task Exception_With_Serializable_Data_Propagates_To_Client()
     {
@@ -89,7 +80,7 @@ public async Task Exception_With_Serializable_Data_Propagates_To_Client()
         Assert.Equal((McpErrorCode)(-32002), exception.ErrorCode);
 
         // Skip data verification on .NET Framework since JsonElement cannot be stored in Exception.Data
-        if (IsNetFramework)
+        if (PlatformDetection.IsNetFramework)
         {
             return;
         }
@@ -132,7 +123,7 @@ public async Task Exception_With_NonSerializable_Data_Still_Propagates_Error_To_
         Assert.Equal((McpErrorCode)(-32002), exception.ErrorCode);
 
         // Skip data verification on .NET Framework since JsonElement cannot be stored in Exception.Data
-        if (IsNetFramework)
+        if (PlatformDetection.IsNetFramework)
         {
             return;
         }

tests/ModelContextProtocol.Tests/PlatformDetection.cs

@@ -6,4 +6,13 @@ internal static class PlatformDetection
 {
     public static bool IsMonoRuntime { get; } = Type.GetType("Mono.Runtime") is not null;
     public static bool IsWindows { get; } = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
+
+    // On .NET Framework, Exception.Data requires values to be serializable with [Serializable].
+    // JsonElement is not marked as serializable, so certain features are not available on that platform.
+    public static bool IsNetFramework { get; } =
+#if NET
+        false;
+#else
+        true;
+#endif
 }