Force stdin/stdout encoding to UTF-8

The character encoding of the stdin/stdout streams in Python is platform-
dependent. On Windows it will be something weird, like CP437 or CP1252,
depending on the locale. This change ensures that no matter the platform,
UTF-8 is used.

src/mcp/server/stdio.py

@@ -20,6 +20,7 @@ async def run_server():
 
 import sys
 from contextlib import asynccontextmanager
+from io import TextIOWrapper
 
 import anyio
 import anyio.lowlevel
@@ -38,11 +39,13 @@ async def stdio_server(
     from the current process' stdin and writing to stdout.
     """
     # Purposely not using context managers for these, as we don't want to close
-    # standard process handles.
+    # standard process handles. Encoding of stdin/stdout as text streams on
+    # 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(sys.stdin)
+        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8"))
     if not stdout:
-        stdout = anyio.wrap_file(sys.stdout)
+        stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
 
     read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception]
     read_stream_writer: MemoryObjectSendStream[types.JSONRPCMessage | Exception]