test: cover transformer output format validation

[gaoflow]

Summary

• add transformer detector tests for both supported output formats on predict() and predict_prompt()
• add a regression test that unsupported transformer output_format values raise a clear ValueError

Notes

• Runtime validation for transformer and LLM output_format is already present on current main; this fills the missing transformer test coverage for Validate output_format and raise a clear error on invalid values #46.

Closes #46

Tests

• .venv/bin/python -m pytest tests/test_inference_pytest.py -q -k output_format
• .venv/bin/python -m pytest tests/test_inference_pytest.py -q
• .venv/bin/python -m ruff format --check tests/test_inference_pytest.py
• .venv/bin/python -m ruff check tests/test_inference_pytest.py