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