fix: handle non-UTF-8 bytes in stdio server stdin (#2302)

src/mcp/server/stdio.py

@@ -39,7 +39,7 @@ 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"))
+        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
     if not stdout:
         stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
 
@@ -58,7 +58,7 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
                 async for line in stdin:
                     try:
                         message = types.jsonrpc_message_adapter.validate_json(line, by_name=False)
-                    except Exception as exc:  # pragma: no cover
+                    except Exception as exc:
                         await read_stream_writer.send(exc)
                         continue