Add dedicated Extensions constants to Experimentals class

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

Author: Copilot

src/Common/Experimentals.cs

@@ -35,4 +35,19 @@ internal static class Experimentals
     /// URL for the experimental MCP Tasks feature.
     /// </summary>
     public const string Tasks_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp001";
+
+    /// <summary>
+    /// Diagnostic ID for the experimental MCP Extensions feature.
+    /// </summary>
+    public const string Extensions_DiagnosticId = "MCPEXP001";
+
+    /// <summary>
+    /// Message for the experimental MCP Extensions feature.
+    /// </summary>
+    public const string Extensions_Message = "The Extensions feature is experimental per the MCP specification and is subject to change.";
+
+    /// <summary>
+    /// URL for the experimental MCP Extensions feature.
+    /// </summary>
+    public const string Extensions_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp001";
 }

src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs

@@ -100,6 +100,6 @@ public sealed class ClientCapabilities
     /// </para>
     /// </remarks>
     [JsonPropertyName("extensions")]
-    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)] // SEP-2133: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133
+    [Experimental(Experimentals.Extensions_DiagnosticId, UrlFormat = Experimentals.Extensions_Url)] // SEP-2133: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133
     public IDictionary<string, object>? Extensions { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs

@@ -99,6 +99,6 @@ public sealed class ServerCapabilities
     /// </para>
     /// </remarks>
     [JsonPropertyName("extensions")]
-    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)] // SEP-2133: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133
+    [Experimental(Experimentals.Extensions_DiagnosticId, UrlFormat = Experimentals.Extensions_Url)] // SEP-2133: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133
     public IDictionary<string, object>? Extensions { get; set; }
 }