helper: cast openat mode for darwin

mode_t is u16 on macOS and a variadic argument promotes to at least c_uint, so aarch64-apple-darwin failed E0617 while every Linux target built clean.

The cast is a no-op where mode_t is already u32.

helper/src/main.rs

@@ -885,7 +885,7 @@ fn open_agent_exchange_file(
             directory.as_raw_fd(),
             name.as_ptr(),
             flags | libc::O_NOFOLLOW | libc::O_CLOEXEC,
-            mode,
+            mode as libc::c_uint,
         )
     };
     if descriptor < 0 {