Pydantic deprecation warning: class-based config should use ConfigDict

Description

When running tests with Pydantic 2.x, we get deprecation warnings:

PydanticDeprecatedSince20: Support for class-based `config` is deprecated, use ConfigDict instead. 
Deprecated in Pydantic V2.0 to be removed in V3.0. 
See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.11/migration/

Location

The warning occurs at:
/policyengine_core/commons/formulas.py (exact location needs to be determined)

Impact

• Currently just a warning
• Will break when Pydantic 3.0 is released
• Shows up in all test runs

Solution

Update any Pydantic models using class-based config to use ConfigDict instead.

Example migration:

# Old style
class MyModel(BaseModel):
    class Config:
        arbitrary_types_allowed = True

# New style
from pydantic import ConfigDict

class MyModel(BaseModel):
    model_config = ConfigDict(arbitrary_types_allowed=True)

Found in

PolicyEngine UK tests running on Python 3.13 with policyengine-core 3.19.3

Start with policyengine_core/commons/formulas.py and identify the Pydantic models using class-based Config. Consult the linked Pydantic V2 migration guidance, update each affected model to ConfigDict, and run the test suite to confirm the deprecation warnings are gone.