tests: add official ONNX file list check

[strimo378]

Motivation

• Provide a single-source manifest of the ONNX backend test data to detect accidental additions or removals.
• Ensure the repository's baked-in list of official ONNX test files matches the on-disk onnx-org test data tree.
• Make it easy to spot when the upstream ONNX test data set changes so maintainers can update CI or manifests.

Description

• Add tests/test_official_onnx_files.py which defines OFFICIAL_ONNX_FILES containing a sorted list of relative .onnx paths from the ONNX backend test data.
• Implement _collect_onnx_files(data_root: Path) to gather .onnx files recursively and compare that set against the baked-in manifest.
• Add test_official_onnx_files() which asserts there are no missing or extra files and reports any mismatches.

Testing

• Ran pytest tests/test_official_onnx_files.py which collected and executed the new test and reported 1 passed (success).
• Attempting pytest -k test_official_onnx_files failed during global collection due to missing/incorrect ONNX test dependencies (onnx.onnx_cpp2py_export / parameterized), not a failure of the new test itself.

---

Codex Task