Revert StreamClientSessionTransport refactoring to fix CI failure

The refactoring to bypass TextWriter and write directly to the
underlying Stream changed the timing of pipe writes, causing the
StdioClientTransportTests.CreateAsync_ValidProcessInvalidServer_StdErrCallbackInvoked
test to fail on macOS and Ubuntu CI. Reverting to keep the original
TextWriter-based write path while still using the deferred logging
overload to avoid unnecessary serialization when trace logging is
disabled.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

@@ -8,12 +8,10 @@ namespace ModelContextProtocol.Client;
 /// <summary>Provides the client side of a stream-based session transport.</summary>
 internal class StreamClientSessionTransport : TransportBase
 {
-    private static readonly byte[] s_newlineBytes = "\n"u8.ToArray();
-
     internal static UTF8Encoding NoBomUtf8Encoding { get; } = new(encoderShouldEmitUTF8Identifier: false);
 
     private readonly TextReader _serverOutput;
-    private readonly Stream _serverInputStream;
+    private readonly TextWriter _serverInput;
     private readonly SemaphoreSlim _sendLock = new(1, 1);
     private CancellationTokenSource? _shutdownCts = new();
     private Task? _readTask;
@@ -22,13 +20,12 @@ internal class StreamClientSessionTransport : TransportBase
     /// Initializes a new instance of the <see cref="StreamClientSessionTransport"/> class.
     /// </summary>
     /// <param name="serverInput">
-    /// The server's input stream. Messages written to this stream will be sent to the server.
+    /// The text writer connected to the server's input stream.
+    /// Messages written to this writer will be sent to the server.
     /// </param>
     /// <param name="serverOutput">
-    /// The server's output stream. Messages read from this stream will be received from the server.
-    /// </param>
-    /// <param name="encoding">
-    /// The encoding used for reading and writing messages from the input and output streams. Defaults to UTF-8 without BOM if null.
+    /// The text reader connected to the server's output stream.
+    /// Messages read from this reader will be received from the server.
     /// </param>
     /// <param name="endpointName">
     /// A name that identifies this transport endpoint in logs.
@@ -40,18 +37,12 @@ internal class StreamClientSessionTransport : TransportBase
     /// This constructor starts a background task to read messages from the server output stream.
     /// The transport will be marked as connected once initialized.
     /// </remarks>
-    public StreamClientSessionTransport(Stream serverInput, Stream serverOutput, Encoding? encoding, string endpointName, ILoggerFactory? loggerFactory)
+    public StreamClientSessionTransport(
+        TextWriter serverInput, TextReader serverOutput, string endpointName, ILoggerFactory? loggerFactory)
         : base(endpointName, loggerFactory)
     {
-        Throw.IfNull(serverInput);
-        Throw.IfNull(serverOutput);
-
-        _serverInputStream = serverInput;
-#if NET
-        _serverOutput = new StreamReader(serverOutput, encoding ?? NoBomUtf8Encoding);
-#else
-        _serverOutput = new CancellableStreamReader(serverOutput, encoding ?? NoBomUtf8Encoding);
-#endif
+        _serverOutput = serverOutput;
+        _serverInput = serverInput;
 
         SetConnected();
 
@@ -66,6 +57,43 @@ public StreamClientSessionTransport(Stream serverInput, Stream serverOutput, Enc
         readTask.Start();
     }
 
+    /// <summary>
+    /// Initializes a new instance of the <see cref="StreamClientSessionTransport"/> class.
+    /// </summary>
+    /// <param name="serverInput">
+    /// The server's input stream. Messages written to this stream will be sent to the server.
+    /// </param>
+    /// <param name="serverOutput">
+    /// The server's output stream. Messages read from this stream will be received from the server.
+    /// </param>
+    /// <param name="encoding">
+    /// The encoding used for reading and writing messages from the input and output streams. Defaults to UTF-8 without BOM if null.
+    /// </param>
+    /// <param name="endpointName">
+    /// A name that identifies this transport endpoint in logs.
+    /// </param>
+    /// <param name="loggerFactory">
+    /// Optional factory for creating loggers. If null, a NullLogger is used.
+    /// </param>
+    /// <remarks>
+    /// This constructor starts a background task to read messages from the server output stream.
+    /// The transport will be marked as connected once initialized.
+    /// </remarks>
+    public StreamClientSessionTransport(Stream serverInput, Stream serverOutput, Encoding? encoding, string endpointName, ILoggerFactory? loggerFactory)
+        : this(
+            new StreamWriter(serverInput, encoding ?? NoBomUtf8Encoding),
+#if NET
+            new StreamReader(serverOutput, encoding ?? NoBomUtf8Encoding),
+#else
+            new CancellableStreamReader(serverOutput, encoding ?? NoBomUtf8Encoding),
+#endif
+            endpointName,
+            loggerFactory)
+    {
+        Throw.IfNull(serverInput);
+        Throw.IfNull(serverOutput);
+    }
+
     /// <inheritdoc/>
     public override async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
     {
@@ -77,13 +105,14 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
 
         LogTransportSendingMessageSensitive(message);
 
+        var json = JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
+
         using var _ = await _sendLock.LockAsync(cancellationToken).ConfigureAwait(false);
         try
         {
-            var json = JsonSerializer.SerializeToUtf8Bytes(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
-            await _serverInputStream.WriteAsync(json, cancellationToken).ConfigureAwait(false);
-            await _serverInputStream.WriteAsync(s_newlineBytes, cancellationToken).ConfigureAwait(false);
-            await _serverInputStream.FlushAsync(cancellationToken).ConfigureAwait(false);
+            // Write the message followed by a newline using our UTF-8 writer
+            await _serverInput.WriteLineAsync(json).ConfigureAwait(false);
+            await _serverInput.FlushAsync(cancellationToken).ConfigureAwait(false);
         }
         catch (Exception ex)
         {