Add explicit [Experimental] protected constructors to McpClient and McpServer (#1363)

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: jeffhandley <1031940+jeffhandley@users.noreply.github.com>
Co-authored-by: Jeff Handley <jeffhandley@users.noreply.github.com>

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -1,4 +1,5 @@
-using ModelContextProtocol.Protocol;
+using System.Diagnostics.CodeAnalysis;
+using ModelContextProtocol.Protocol;
 
 namespace ModelContextProtocol.Client;
 
@@ -7,6 +8,14 @@ namespace ModelContextProtocol.Client;
 /// </summary>
 public abstract partial class McpClient : McpSession
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpClient"/> class.
+    /// </summary>
+    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    protected McpClient()
+    {
+    }
+
     /// <summary>
     /// Gets the capabilities supported by the connected server.
     /// </summary>

src/ModelContextProtocol.Core/Client/McpClientImpl.cs

@@ -6,6 +6,7 @@ using System.Text.Json;
 namespace ModelContextProtocol.Client;
 
 /// <inheritdoc/>
+#pragma warning disable MCPEXP002
 internal sealed partial class McpClientImpl : McpClient
 {
     private static Implementation DefaultImplementation { get; } = new()
@@ -37,6 +38,7 @@ internal sealed partial class McpClientImpl : McpClient
     /// <param name="options">Options for the client, defining protocol version and capabilities.</param>
     /// <param name="loggerFactory">The logger factory.</param>
     internal McpClientImpl(ITransport transport, string endpointName, McpClientOptions? options, ILoggerFactory? loggerFactory)
+#pragma warning restore MCPEXP002
     {
         options ??= new();