More string intermediates in read side if MCP transport

[ericstj]

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)