Fix stdio transport closing sys.stdin.buffer/sys.stdout.buffer on exit

When using transport='stdio', anyio.wrap_file() wraps a TextIOWrapper
around sys.stdin.buffer/sys.stdout.buffer. When the server exits and the
async file is closed, TextIOWrapper.close() propagates to close the
underlying buffer, making subsequent stdio operations fail with:
  ValueError: underlying buffer has been closed

This adds a _DetachingTextIOWrapper that detaches from the buffer on
close, preventing the propagation. The original comment in the code
stated 'Purposely not using context managers for these, as we don't want
to close standard process handles' — this fix aligns the implementation
with that intent.

Fixes #1933

Author: TheChyeahhh

src/mcp/server/stdio.py

@@ -21,6 +21,17 @@ async def run_server():
 from contextlib import asynccontextmanager
 from io import TextIOWrapper
 
+
+class _DetachingTextIOWrapper(TextIOWrapper):
+    """TextIOWrapper that detaches from its buffer on close, preventing
+    the underlying buffer (e.g. sys.stdin.buffer) from being closed."""
+
+    def close(self):
+        try:
+            self.detach()
+        except (ValueError, OSError):
+            pass
+
 import anyio
 import anyio.lowlevel
 
@@ -39,9 +50,9 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
     # python is platform-dependent (Windows is particularly problematic), so we
     # re-wrap the underlying binary stream to ensure UTF-8.
     if not stdin:
-        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
+        stdin = anyio.wrap_file(_DetachingTextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
     if not stdout:
-        stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
+        stdout = anyio.wrap_file(_DetachingTextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
 
     read_stream_writer, read_stream = create_context_streams[SessionMessage | Exception](0)
     write_stream, write_stream_reader = create_context_streams[SessionMessage](0)