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/Server/DestinationBoundMcpServer.cs

@@ -3,7 +3,9 @@ using System.Diagnostics;
 
 namespace ModelContextProtocol.Server;
 
+#pragma warning disable MCPEXP002
 internal sealed class DestinationBoundMcpServer(McpServerImpl server, ITransport? transport) : McpServer
+#pragma warning restore MCPEXP002
 {
     public override string? SessionId => transport?.SessionId ?? server.SessionId;
     public override string? NegotiatedProtocolVersion => server.NegotiatedProtocolVersion;

src/ModelContextProtocol.Core/Server/McpServer.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using ModelContextProtocol.Protocol;
 
 namespace ModelContextProtocol.Server;
@@ -7,6 +8,14 @@ namespace ModelContextProtocol.Server;
 /// </summary>
 public abstract partial class McpServer : McpSession
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpServer"/> class.
+    /// </summary>
+    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    protected McpServer()
+    {
+    }
+
     /// <summary>
     /// Gets the capabilities supported by the client.
     /// </summary>

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -9,6 +9,7 @@ using System.Text.Json.Serialization.Metadata;
 namespace ModelContextProtocol.Server;
 
 /// <inheritdoc />
+#pragma warning disable MCPEXP002
 internal sealed partial class McpServerImpl : McpServer
 {
     internal static Implementation DefaultImplementation { get; } = new()
@@ -54,6 +55,7 @@ internal sealed partial class McpServerImpl : McpServer
     /// <param name="serviceProvider">Optional service provider to use for dependency injection</param>
     /// <exception cref="McpException">The server was incorrectly configured.</exception>
     public McpServerImpl(ITransport transport, McpServerOptions options, ILoggerFactory? loggerFactory, IServiceProvider? serviceProvider)
+#pragma warning restore MCPEXP002
     {
         Throw.IfNull(transport);
         Throw.IfNull(options);