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>

tests/ModelContextProtocol.Tests/Client/McpClientFactoryTests.cs

@@ -187,7 +187,7 @@ public class McpClientFactoryTests
         await Assert.ThrowsAsync<ArgumentException>(() => McpClientFactory.CreateAsync(config, _defaultOptions, cancellationToken: TestContext.Current.CancellationToken));
     }
 
-    private sealed class NopTransport : IClientTransport
+    private sealed class NopTransport : ITransport, IClientTransport
     {
         private readonly Channel<IJsonRpcMessage> _channel = Channel.CreateUnbounded<IJsonRpcMessage>();
 
@@ -195,8 +195,7 @@ public class McpClientFactoryTests
 
         public ChannelReader<IJsonRpcMessage> MessageReader => _channel.Reader;
 
-        public Task ConnectAsync(CancellationToken cancellationToken = default) =>
-            Task.CompletedTask;
+        public Task<ITransport> ConnectAsync(CancellationToken cancellationToken = default) => Task.FromResult<ITransport>(this);
 
         public ValueTask DisposeAsync() => default;