Add Description property to Implementation type per MCP spec (#966)

* Initial plan

* Add Description property to Implementation type

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

* Add test to validate client Description from server perspective

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol.Core/Protocol/Implementation.cs

@@ -37,6 +37,22 @@ public sealed class Implementation : IBaseMetadata
     [JsonPropertyName("version")]
     public required string Version { get; set; }
 
+    /// <summary>
+    /// Gets or sets an optional description of the implementation.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This description helps users and developers understand what the implementation provides
+    /// and its purpose. It should clearly explain the functionality and capabilities offered.
+    /// </para>
+    /// <para>
+    /// The description is typically used in documentation, UI displays, and for providing context
+    /// to users about the server or client they are interacting with.
+    /// </para>
+    /// </remarks>
+    [JsonPropertyName("description")]
+    public string? Description { get; set; }
+
     /// <summary>
     /// Gets or sets an optional list of icons for this implementation.
     /// </summary>