tests/test_fastapi/test_fastapi_usage.py::test_read_main fails with Pydantic 2.13

[cjwatson]

Describe the bug
tests/test_fastapi/test_fastapi_usage.py::test_read_main fails with Pydantic 2.13.

To Reproduce
In a clone of ormar:

1. poetry install -E all
2. `poetry add 'pydantic>=2.13'
3. .venv/bin/pytest tests/test_fastapi/test_fastapi_usage.py -vv

FAILED tests/test_fastapi/test_fastapi_usage.py::test_read_main - AssertionError: assert {'id': 1, 'name': 'test', 'category': {'id': None, 'name': 'test cat'}} == {'category': {'id': None, 'items': [{'category': {'id': None, 'name': 'test cat'}, 'id': 1, 'name': 'test'}], 'name': 'test cat'}, 'id': 1, 'name': 'test'}

  Common items:
  {'id': 1, 'name': 'test'}
  Differing items:
  {'category': {'id': None, 'name': 'test cat'}} != {'category': {'id': None, 'items': [{'category': {'id': None, 'name': 'test cat'}, 'id': 1, 'name': 'test'}], 'name': 'test cat'}}

  Full diff:
    {
        'category': {
            'id': None,
  -         'items': [
  -             {
  -                 'category': {
  -                     'id': None,
  -                     'name': 'test cat',
  -                 },
  -                 'id': 1,
  -                 'name': 'test',
  -             },
  -         ],
            'name': 'test cat',
        },
        'id': 1,
        'name': 'test',
    }

Expected behavior
Passing tests.

Versions (please complete the following information):
All versions as in current poetry.lock, except for pydantic-core 2.46.4 and pydantic 2.13.4.

Additional context
I realize that this isn't what the Poetry lock file specifies, but Debian unstable currently has pydantic 2.13.3 and we're running into this in distribution-wide tests there. It's also visible in #1660.

I tracked this down as far as pydantic.type_adapter.TypeAdapter.dump_json. Setting a breakpoint there, I find that instance is something along the lines of Item({'id': 1, 'name': 'test', 'category': Category({'id': None, 'name': 'test cat', 'items': [<weakproxy at 0x7f92342dbf60; to 'tests.test_fastapi.test_fastapi_usage.Item' at 0x7f92342e04b0>]})}). pydantic 2.12.5 serializes this as ormar's tests expect, but pydantic 2.13.4 drops the items. But I'm not sure whether this is because of a change in the handling of weakref proxies, or because something is different about the instance's pydantic metadata. Hopefully you can figure it out?