Fix Pydantic v2 deprecation warning in json_adapter

[brishin]

Replace deprecated __config__ parameter with model_config in pydantic.create_model() to remove one case of the PydanticDeprecatedSince20 warning.

The warning was:

.venv/lib/python3.13/site-packages/pydantic/_internal/_config.py:323
  /Users/brian/Programming/dspy/.venv/lib/python3.13/site-packages/pydantic/_internal/_config.py:323: 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/
    warnings.warn(DEPRECATION_MESSAGE, DeprecationWarning)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

This change updates the code to use the Pydantic v2 approach.

[okhat]

Thanks so much @brishin , mind checking the failed tests?