Cannot pickle Structs with cloudpickle

Describe the bug
csp.Structs don't work with cloudpickle.

To Reproduce

import csp
import cloudpickle

class C(csp.Struct):
    a: int = 1
    b: str = ""

c = C()
cloudpickle.loads(cloudpickle.dumps(c))

Expected behavior

Pickling and unpickling should work correctly.

Error Message

Traceback (most recent call last):
  File "/data01/ac14141/github/ray/csp/t.py", line 9, in <module>
    cloudpickle.loads(cloudpickle.dumps(c))
  File "/data01/ac14141/.venvs/csp-ray/lib/python3.12/site-packages/cloudpickle/cloudpickle.py", line 553, in _make_skeleton_class
    skeleton_class = types.new_class(
                     ^^^^^^^^^^^^^^^^
  File "/data01/ac14141/.venvs/csp-ray/lib/python3.12/types.py", line 74, in new_class
    return meta(name, resolved_bases, ns, **kwds)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/data01/ac14141/github/ray/csp/csp/impl/struct.py", line 83, in __new__
    res = super().__new__(cls, name, bases, dct)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Struct.cpp:StructMeta:49:TypeError: Struct types must define at least 1 field

Runtime Environment

import sys, csp; print(csp.__version__); print(sys.version); print(sys.platform)
0.14.0
3.12.12 | packaged by conda-forge | (main, Jan 26 2026, 23:51:32) [GCC 14.3.0]
linux

Additional context

Run the supplied Python cloudpickle round-trip first, then inspect csp/impl/struct.py around the Struct metaclass new call at line 83. Done means the provided C instance can be serialized and deserialized with cloudpickle without the Struct types must define at least 1 field error.