Migrate front from pydantic v1 to pydantic v2

[thorwhalen]

Closes #30.

Why

front pinned pydantic==1.10.12, propagating that pin into streamlitfront,
extrude, and anything else composing front elements. Meanwhile, newer
i2mint packages (e.g. ju via http2py) call pydantic v2 APIs at import
time, so the v1 pin was breaking the wider ecosystem. This PR cuts the
anchor: front now requires pydantic>=2.

This is a hard cut — no v1 compatibility shim. The migration surface
inside front is tiny (one module, ~140 lines), and dual-supporting v1
and v2 would cost more than the migration itself.

What changed

front/py2pydantic.py

• Doctest examples updated from schema_json() to model_json_schema()
  (v1's schema_json() is removed in v2).
• func_to_pyd_model_specs now infers the field type from a default's
  type when no annotation is provided. Pydantic v2 requires explicit
  (type, default) tuples; v1's "bare default infers type" path no
  longer works.
• BaseModel-shadowing field names (e.g. copy, schema) are detected
  proactively now. In v1 a conflict raised NameError, which the code
  caught and used as a trigger for the auto-uppercase rename. In v2,
  pydantic merely warns and leaves the conflict in place, so the
  detection has to be upfront via set(Sig(func).names) & set(dir(BaseModel)).
• Module docstring expanded — it's auto-extracted for docs.

front/tests/test_py2pydantic.py

• model.__fields__ → model.model_fields
• field.type_ → field.annotation
• Defaults are now pydantic_core.PydanticUndefined for required fields
  rather than None. Test now uses field.is_required() to check.

front/tests/test_py2pydantic_integration.py (new)

End-to-end coverage of the path that downstream packages actually
depend on:

• generate input model from a Python function
• instantiate (exercises validation/coercion)
• dispatch through the wrapper
• verify the original function ran and returned the right thing
• verify ValidationError surfaces on bad input

This was previously only exercised by the doctest. The pre-existing
unit test only checked model-class structure, not dispatch behavior.

pyproject.toml

• pydantic==1.10.12 → pydantic>=2

Migration guide for downstream callers

If your code uses front.py2pydantic.func_to_pyd_input_model_cls(...):

pydantic v1	pydantic v2
model.__fields__	model.model_fields
field.type_	field.annotation
field.default (None if required)	field.is_required() + field.default
model.schema_json()	json.dumps(model.model_json_schema())
instance.dict()	instance.model_dump()
instance.json()	instance.model_dump_json()

create_model, instantiating models, and basic validation are unchanged.

Verified

$ python -m pytest --doctest-modules front/ tests/ \
    --ignore=front/scrap --ignore=front/examples --ignore=examples \
    --ignore=scrap --ignore=docsrc
======================== 37 passed, 2 warnings in 0.69s ========================

Including the new integration tests, all 37 doctests + unit tests pass
under pydantic 2.12.2.

Follow-ups

After this lands and a new front version is on PyPI:

1. streamlitfront — drop its v1-era streamlit_pydantic / pydantic
   usage, bump to a v2-compatible release. Separate PR.
2. extrude — revert the temporary CI workarounds (Python 3.12 +
   Windows skip) added during the wider CI sweep. Should be one-line
   reverts once the ecosystem is consistent on v2.