fix: handle missing pydantic-core corner case

[elventear]

This change handles a corner case where pydantic is installed in the environment but pydantic-core is missing. The current code assumes that if pydantic is importable, then pydantic is available, but the access of pydantic.BaseModel does a lazy import of pydantic-core which is not handled, crashing the application that is using cappa.

To handle the issue, BaseModel is imported eagerly during the detection step, which will throw an error when pydantic-core is not available.

I have observed this in the wild, where I am not using pydantic with cappa at all, nor it is one of my direct dependencies, but it is still being pulled transitively from other sources, crashing my application. This issue surfaced with https://github.com/DanCardin/cappa/releases/tag/v0.19.0.

[DanCardin]

I'm happy to accept this change, as I see no harm it in.

But besides that, I wonder if you're befallen by something like python-poetry/poetry#9266. If you have pydantic you should have pydantic_core. I recently found myself hitting related-sounding issues where the lock wasn't installing required transitive dependencies (including pydantic_core).

(Not that it's necessarily relevant to you that pydantic is broken, if you're not using it. Just that you might have other issues, if this is the same problem I encountered)