fix: access model_fields from class instead of instance

[sene03]

Fixes deprecation warning in Pydantic 2.11+ where accessing model_fields on an instance is deprecated. Changed to use type(model).model_fields in model_to_dict().

Closes #652

Changes

How I tested this

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[skrawcz]

@sene03 I think we need a minimum version pin in pyproject.toml with this change -- do you know what that would be?

[skrawcz]

you need to pair this with #654

[andreahlert]

Tested locally with pydantic 2.12.5. This is the correct and minimal fix - model_to_dict is the only function that accesses model_fields on an instance. The other functions (subset_model, model_from_state, _validate_keys) already receive classes via Type[BaseModel] and never triggered the deprecation. Clean fix, no unnecessary changes.

Re @skrawcz's comment about version pins: type(model).model_fields works on all pydantic 2.x - no minimum bump is strictly required for this fix. If a pin is desired for other reasons, PR #654 already handles that separately, which is the right separation of concerns.

One suggestion: add a behavioral test that asserts no DeprecationWarning is emitted from model_to_dict, rather than just testing the output. This would catch regressions if someone reverts to instance access:

import warnings

def test_model_to_dict_no_deprecation_warning():
    model = OriginalModel(foo=1, bar="bar", nested=NestedModel(nested_field1=1))
    with warnings.catch_warnings():
        warnings.simplefilter("error")
        result = model_to_dict(model)
    assert "foo" in result