Add PATHEXT to default STDIO env vars in windows (#1256)

Author: timesler

src/mcp/client/stdio/__init__.py

@@ -32,6 +32,7 @@
         "HOMEPATH",
         "LOCALAPPDATA",
         "PATH",
+        "PATHEXT",
         "PROCESSOR_ARCHITECTURE",
         "SYSTEMDRIVE",
         "SYSTEMROOT",