Fix ValidatorIterator serialization

[ndtretyak]

What was changed

to_jsonable_python is now used instead of default=str to serialize pydantic models.

Why?

Without this change ValidatorIterator is serialized as "ValidatorIterator(...)"

Checklist

1. Closes

2. How was this tested:

Unit tests

3. Any docs updates needed?

[tconley1428]

@ndtretyak if you would like to resolve the merge conflicts and linting issues (See the readme or poe lint), then we can take the PR. Otherwise I can take the changes in and fix them, but your name will be lost.