Add MCP Apps extension support (typed metadata, attribute, and helpers) (#1484)

src/ModelContextProtocol.Extensions.Apps/Server/McpUiClientCapabilities.cs

@@ -0,0 +1,26 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Extensions.Apps;
+
+/// <summary>
+/// Represents the MCP Apps capabilities advertised by a client.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This object is the value associated with the <c>"io.modelcontextprotocol/ui"</c> key in the
+/// <see cref="Protocol.ClientCapabilities.Extensions"/> dictionary.
+/// </para>
+/// </remarks>
+[Experimental(Experimentals.Apps_DiagnosticId, UrlFormat = Experimentals.Apps_Url)]
+public sealed class McpUiClientCapabilities
+{
+    /// <summary>
+    /// Gets or sets the list of MIME types supported by the client for MCP App UI resources.
+    /// </summary>
+    /// <remarks>
+    /// A client that supports MCP Apps must include <c>"text/html;profile=mcp-app"</c> in this list.
+    /// </remarks>
+    [JsonPropertyName("mimeTypes")]
+    public IList<string>? MimeTypes { get; set; }
+}