fix: serialize split_class in split multipart requests

[pandego]

Closes #76

Summary

• serialize split_class as a JSON string before sending multipart split requests
• accept either the documented JSON string form or the typed iterable form at the client boundary
• add a focused multipart regression test for the split payload shape

Testing

• uv run --with pytest --with pytest-xdist --with pytest-asyncio --with respx pytest tests/test_client.py -k 'multipart_repeating_array or split_sends_split_class_as_json_string_in_multipart'
• uv run --with ruff ruff check src/landingai_ade/_client.py src/landingai_ade/types/client_split_params.py tests/test_client.py