pydantic pinning

[pwalczysko]

See discussion on #543

Pydantic is pinned now in the testing suite in

requirements/requirements-test.txt to

pydantic==2.12.5

Note that the test with the pre-release pydantic==2.13.0b2 would fail with

pydantic.errors.PydanticUserError: `Image` is not fully defined; you should define all referenced types, then call `Image.model_rebuild()

We probably need to keep an eye on this, especially when new pydantic is released and/or this repo is released.

cc @will-moore @dstansby