Support defaults from Pydantic fields

[jinnovation]

Contributes to #793.

This PR implements quick-and-dirty support for Pydantic-style default values.

Open questions:

• Should Pydantic support for pdoc be split out into a separate "plugin" package, e.g. pdoc-pydantic or pdoc[pydantic]?
• Is this the best place for default-value-extraction logic for Pydantic classes to live?

[mhils]

Thanks!

Should Pydantic support for pdoc be split out into a separate "plugin" package, e.g. pdoc-pydantic or pdoc[pydantic]?

Too much complexity for now. I think pydantic is popular enough to have support for it builtin for now.

Is this the best place for default-value-extraction logic for Pydantic classes to live?

Pragmatically yes. See my comments below on how we can keep things reasonably clean.

[mhils]

Let's move this logic into pdoc/_pydantic.py and shorten it here to something like this:

Suggested change

	try:
	default_value = self._member_objects["__pydantic_fields__"][
	name
	].default
	except KeyError:
	default_value = obj
	default_value = _pydantic.default_value(self, name, obj)

This allows us to keep all pydantic-specific logic in a central location if we ever decide to break it out.

[jinnovation]

SGTM

[mhils]

Please add a snapshot test instead:

1. Add Python file to test/testdata, e.g. test/testdata/pydantic.py
2. Register it here.
3. uv run test/test_snapshot.py

[jinnovation]

Sounds good.

[jinnovation]

Generating snapshot tests appears to be blocked on the same issue as #792.