Preserve required argument order in errors

[tandede]

Summary

• preserve each required_args variant's declaration order when listing missing arguments
• add an exact regression assertion for multi-argument error messages

Why

The single-variant error path currently computes missing arguments with a set difference. Set iteration depends on Python's randomized hash seed, so an identical invalid SDK call can list parameters in a different order across processes. For example, the same declaration produced both 'model', 'messages' or 'stream' and 'messages', 'stream' or 'model' under different PYTHONHASHSEED values.

Filtering the declared variant in place makes the message deterministic and keeps it aligned with the method signature.

Validation

• verified identical output across 8 different PYTHONHASHSEED values
• python -m pytest -q -n 0 tests/test_required_args.py tests/test_utils tests/test_models.py (197 passed)
• ruff check .
• ruff format --check .
• mypy src/openai/_utils/_utils.py