Improve exception diagnostics for stdio client (#376)

* Improve exception diagnostics for stdio client

Track the last several lines of stderr and include those in an exception created as part of CleanupAsync if the server process has already exited. This is based on an assumption that the server should never exit prior to CleanupAsync being called.

* Address feedback and fix test failure

tests/ModelContextProtocol.Tests/Transport/StdioClientTransportTests.cs

@@ -0,0 +1,21 @@
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Protocol.Transport;
+using System.Runtime.InteropServices;
+
+namespace ModelContextProtocol.Tests.Transport;
+
+public class StdioClientTransportTests
+{
+    [Fact]
+    public async Task CreateAsync_ValidProcessInvalidServer_Throws()
+    {
+        string id = Guid.NewGuid().ToString("N");
+
+        StdioClientTransport transport = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ?
+            new(new() { Command = "cmd", Arguments = ["/C", $"echo \"{id}\" >&2"] }) :
+            new(new() { Command = "ls", Arguments = [id] });
+
+        IOException e = await Assert.ThrowsAsync<IOException>(() => McpClientFactory.CreateAsync(transport, cancellationToken: TestContext.Current.CancellationToken));
+        Assert.Contains(id, e.ToString());
+    }
+}