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.

Tái hiện các lỗi với Python 3.14.2 bằng cách sử dụng tests/test_inputs.py, sau đó kiểm tra betterproto/__init__.py quanh phần tra cứu siêu dữ liệu ở dòng 909 và phần xử lý kiểu đang gây lỗi gần dòng 1191. Được xem là hoàn thành khi các phép tham số hóa của namespace_builtin_types đều vượt qua và package được build cũng như các test chạy thành công trên Python 3.14.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.