Refactor transports to help enable graceful shutdown (#142)

- This also paves the way for better multi-session support
- We should definitely rethink names for the transport API
- For now, I kept the names similar as possible, so we can
  focus on the API shape

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol/Hosting/McpServerHostedService.cs

@@ -1,31 +0,0 @@
-using ModelContextProtocol.Server;
-using ModelContextProtocol.Utils;
-using Microsoft.Extensions.Hosting;
-
-namespace ModelContextProtocol.Hosting;
-
-/// <summary>
-/// Hosted service for the MCP server.
-/// </summary>
-public class McpServerHostedService : BackgroundService
-{
-    private readonly IMcpServer _server;
-
-    /// <summary>
-    /// Creates a new instance of the McpServerHostedService.
-    /// </summary>
-    /// <param name="server">The MCP server instance</param>
-    /// <exception cref="ArgumentNullException"></exception>
-    public McpServerHostedService(IMcpServer server)
-    {
-        Throw.IfNull(server);
-     
-        _server = server;
-    }
-
-    /// <inheritdoc />
-    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
-    {
-        await _server.StartAsync(stoppingToken).ConfigureAwait(false);
-    }
-}

src/ModelContextProtocol/Hosting/McpServerMultiSessionHostedService.cs

@@ -0,0 +1,43 @@
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using ModelContextProtocol.Protocol.Transport;
+using ModelContextProtocol.Server;
+
+namespace ModelContextProtocol.Hosting;
+
+/// <summary>
+/// Hosted service for a multi-session (i.e. HTTP) MCP server.
+/// </summary>
+internal sealed class McpServerMultiSessionHostedService : BackgroundService
+{
+    private readonly IServerTransport _serverTransport;
+    private readonly McpServerOptions _serverOptions;
+    private readonly ILoggerFactory _loggerFactory;
+    private readonly IServiceProvider _serviceProvider;
+
+    public McpServerMultiSessionHostedService(
+        IServerTransport serverTransport,
+        IOptions<McpServerOptions> serverOptions,
+        ILoggerFactory loggerFactory,
+        IServiceProvider serviceProvider)
+    {
+        _serverTransport = serverTransport;
+        _serverOptions = serverOptions.Value;
+        _loggerFactory = loggerFactory;
+        _serviceProvider = serviceProvider;
+    }
+
+    /// <inheritdoc />
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        while (await AcceptSessionAsync(stoppingToken).ConfigureAwait(false) is { } server)
+        {
+            // TODO: Track all running sessions and wait for all sessions to complete for graceful shutdown.
+            _ = server.RunAsync(stoppingToken);
+        }
+    }
+
+    private Task<IMcpServer> AcceptSessionAsync(CancellationToken cancellationToken)
+        => McpServerFactory.AcceptAsync(_serverTransport, _serverOptions, _loggerFactory, _serviceProvider, cancellationToken);
+}

src/ModelContextProtocol/Hosting/McpServerSingleSessionHostedService.cs

@@ -0,0 +1,13 @@
+using Microsoft.Extensions.Hosting;
+using ModelContextProtocol.Server;
+
+namespace ModelContextProtocol.Hosting;
+
+/// <summary>
+/// Hosted service for a single-session (i.e stdio) MCP server.
+/// </summary>
+internal sealed class McpServerSingleSessionHostedService(IMcpServer session) : BackgroundService
+{
+    /// <inheritdoc />
+    protected override Task ExecuteAsync(CancellationToken stoppingToken) => session.RunAsync(stoppingToken);
+}