fix(stdio): allow configurable memory-stream buffer size

Add read_stream_buffer_size and write_stream_buffer_size parameters to
stdio_server() so callers can increase the internal memory-object-stream
capacity.  The default (0 = synchronous hand-off) is preserved for
backward compatibility.

With max_buffer_size=0 the stdin reader blocks whenever the message
processor is busy, making the server unresponsive to pings and new
requests during slow operations.  A small buffer (e.g. 8) decouples
reading from processing and lets the server remain responsive.

Closes #1333

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

src/mcp/server/stdio.py

@@ -30,7 +30,12 @@ async def run_server():
 
 
 @asynccontextmanager
-async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.AsyncFile[str] | None = None):
+async def stdio_server(
+    stdin: anyio.AsyncFile[str] | None = None,
+    stdout: anyio.AsyncFile[str] | None = None,
+    read_stream_buffer_size: int = 0,
+    write_stream_buffer_size: int = 0,
+):
     """Server transport for stdio: this communicates with an MCP client by reading
     from the current process' stdin and writing to stdout.
     """
@@ -49,8 +54,8 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
     write_stream: MemoryObjectSendStream[SessionMessage]
     write_stream_reader: MemoryObjectReceiveStream[SessionMessage]
 
-    read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
-    write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
+    read_stream_writer, read_stream = anyio.create_memory_object_stream(read_stream_buffer_size)
+    write_stream, write_stream_reader = anyio.create_memory_object_stream(write_stream_buffer_size)
 
     async def stdin_reader():
         try: