mruby-dir: increase buffer size to prevent truncation warning

increase sandbox path buffer from 1024 to 2048 bytes to accommodate
full path with suffix without truncation.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-dir/test/dirtest.c

@@ -31,7 +31,7 @@ make_dir(mrb_state *mrb, const char *name, const char *up)
 mrb_value
 mrb_dirtest_setup(mrb_state *mrb, mrb_value klass)
 {
-  char buf[1024];
+  char buf[2048];
   char cwd[1024];
   const char *aname = "a";
   const char *bname = "b";