Test failures in Python 3.14 due to changes in metadata handling (2.0.0b7)

### Summary

Error: `cls._betterproto_meta` no longer exists @ betterproto/__init__.py:909

### Reproduction Steps

While building release 2.0.0b7 for nixpkgs

### Expected Results

It should build and test cleanly.

### Actual Results

```sh
____________ test_message_can_instantiated[namespace_builtin_types] ____________

cls =

@classproperty
def _betterproto(cls: type[Self]) -> ProtoClassMetadata: # type: ignore
"""
Lazy initialize metadata for each protobuf class.
It may be initialized multiple times in a multi-threaded environment,
but that won't affect the correctness.
"""
try:
> return cls._betterproto_meta
^^^^^^^^^^^^^^^^^^^^^
E AttributeError: type object 'Test' has no attribute '_betterproto_meta'. Did you mean: '_betterproto'?

/nix/store/i3pvlk97zcrmgmcmrjzx1qfgkd8jhkw2-python3.14-betterproto-2.0.0b7/lib/python3.14/site-packages/betterproto/__init__.py:909: AttributeError

During handling of the above exception, another exception occurred:

test_data = TestData(plugin_module= None:
plugin_module, *_ = test_data
> plugin_module.Test()

tests/test_inputs.py:165:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
:18: in __init__
???
/nix/store/i3pvlk97zcrmgmcmrjzx1qfgkd8jhkw2-python3.14-betterproto-2.0.0b7/lib/python3.14/site-packages/betterproto/__init__.py:755: in __post_init__
for field_name, meta in self._betterproto.meta_by_field_name.items():
^^^^^^^^^^^^^^^^^
/nix/store/i3pvlk97zcrmgmcmrjzx1qfgkd8jhkw2-python3.14-betterproto-2.0.0b7/lib/python3.14/site-packages/betterproto/__init__.py:846: in __getattribute__
value = super().__getattribute__(name)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/nix/store/i3pvlk97zcrmgmcmrjzx1qfgkd8jhkw2-python3.14-betterproto-2.0.0b7/lib/python3.14/site-packages/betterproto/utils.py:56: in __get__
return self.__func__(type)
^^^^^^^^^^^^^^^^^^^
/nix/store/i3pvlk97zcrmgmcmrjzx1qfgkd8jhkw2-python3.14-betterproto-2.0.0b7/lib/python3.14/site-packages/betterproto/__init__.py:911: in _betterproto
cls._betterproto_meta = meta = ProtoClassMetadata(cls)
^^^^^^^^^^^^^^^^^^^^^^^
/nix/store/i3pvlk97zcrmgmcmrjzx1qfgkd8jhkw2-python3.14-betterproto-2.0.0b7/lib/python3.14/site-packages/betterproto/__init__.py:691: in __init__
self.default_gen = self._get_default_gen(cls, fields)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/nix/store/i3pvlk97zcrmgmcmrjzx1qfgkd8jhkw2-python3.14-betterproto-2.0.0b7/lib/python3.14/site-packages/betterproto/__init__.py:698: in _get_default_gen
return {field.name: cls._get_field_default_gen(field) for field in fields}
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cls =
field = Field(name='int',type=Field(name='str',type=,default=,default_factory= Any:
t = cls._type_hint(field.name)

is_310_union = isinstance(t, _types_UnionType)
if hasattr(t, "__origin__") or is_310_union:
if is_310_union or t.__origin__ is Union:
# This is an optional field (either wrapped, or using proto3
# field presence). For setting the default we really don't care
# what kind of field it is.
return type(None)
if t.__origin__ is list:
# This is some kind of list (repeated) field.
return list
if t.__origin__ is dict:
# This is some kind of map (dict in Python).
return dict
return t
> if issubclass(t, Enum):
^^^^^^^^^^^^^^^^^^^
E TypeError: issubclass() arg 1 must be a class

/nix/store/i3pvlk97zcrmgmcmrjzx1qfgkd8jhkw2-python3.14-betterproto-2.0.0b7/lib/python3.14/site-packages/betterproto/__init__.py:1191: TypeError.
.
.
=========================== short test summary info ============================
FAILED tests/test_inputs.py::test_message_can_instantiated[namespace_builtin_types] - TypeError: issubclass() arg 1 must be a class
FAILED tests/test_inputs.py::test_message_equality[namespace_builtin_types] - TypeError: issubclass() arg 1 must be a class
FAILED tests/test_inputs.py::test_message_json[namespace_builtin_types] - TypeError: issubclass() arg 1 must be a class

### System Information

Python 3.14.2

### Checklist

- [x] I have searched the issues for duplicates.
- [x] I have shown the entire traceback, if possible.
- [x] I have verified this issue occurs on the latest prelease of betterproto which can be installed using `pip install -U --pre betterproto`, if possible.