Avoid string intermediates in MCP transport read path

[Copilot]

The read side of StreamServerTransport and StreamClientSessionTransport was decoding bytes → string via TextReader.ReadLineAsync(), then re-encoding back to UTF-8 internally during JsonSerializer.Deserialize(string, ...). This round-trip was unnecessary since System.Text.Json can deserialize directly from bytes.

Changes

• Replace TextReader with PipeReader on the input side of both transports. PipeReader.Create(stream) works across all targets; the System.IO.Pipelines dependency was already present.
• Deserialize directly from bytes — single-segment buffers use JsonSerializer.Deserialize(ReadOnlySpan<byte>, typeInfo); multi-segment use new Utf8JsonReader(ReadOnlySequence<byte>, ...) + JsonSerializer.Deserialize(ref reader, typeInfo).
• String conversion only when needed — GetString() (for trace logging) only allocates when Logger.IsEnabled(LogLevel.Trace). For single-segment sequences it calls Encoding.UTF8.GetString(span) directly; multi-segment falls back to ToArray().
• CRLF trimming without slicing — EndsWithCarriageReturn() checks the last byte of the last segment directly rather than creating an intermediate slice.
• Remove encoding constructor parameter from StreamClientSessionTransport — it was only used to configure the now-removed TextReader wrapper. Both call sites already passed null.
• Add Encoding.GetString(ReadOnlySpan<byte>) polyfill to EncodingExtensions.cs for netstandard2.0 compatibility, matching the existing unsafe pattern used by GetByteCount and GetBytes polyfills.

Original prompt

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This section details on the original issue you should resolve

<issue_title>More string intermediates in read side if MCP transport</issue_title>
<issue_description>Similar issue to #1259 which was fixed in #1274.

Read / deserialization is still encoding to a string. We should try to keep as utf8 bytes and deserialize from those:

csharp-sdk/src/ModelContextProtocol.Core/Server/StreamServerTransport.cs

Lines 100 to 116 in 411cef6

	var line = await _inputReader.ReadLineAsync(shutdownToken).ConfigureAwait(false);
	if (string.IsNullOrWhiteSpace(line))
	{
	if (line is null)
	{
	LogTransportEndOfStream(Name);
	break;
	}
	continue;
	}
	LogTransportReceivedMessageSensitive(Name, line);
	try
	{
	if (JsonSerializer.Deserialize(line, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage))) is JsonRpcMessage message)

csharp-sdk/src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

Lines 108 to 121 in 411cef6

	if (await _serverOutput.ReadLineAsync(cancellationToken).ConfigureAwait(false) is not string line)
	{
	LogTransportEndOfStream(Name);
	break;
	}
	if (string.IsNullOrWhiteSpace(line))
	{
	continue;
	}
	LogTransportReceivedMessageSensitive(Name, line);
	await ProcessMessageAsync(line, cancellationToken).ConfigureAwait(false);

csharp-sdk/src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

Lines 140 to 145 in 411cef6

	private async Task ProcessMessageAsync(string line, CancellationToken cancellationToken)
	{
	try
	{
	var message = (JsonRpcMessage?)JsonSerializer.Deserialize(line.AsSpan().Trim(), McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage)));
	if (message != null)

</issue_description>

<agent_instructions>Try to use built in BCL types like PipeReader + ReadOnlySpan callback + System.Text.Json. Avoid adding ifdefs. If a dependency is missing, add it. If we need to polyfill, add the polyfill source for downlevel.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes More string intermediates in read side if MCP transport #1318

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