Pin utf-8 in test source reads (Windows non-UTF-8 locale fix)

[dltsum]

What

Two test helpers read model source files with the locale's preferred encoding:

• _skip_if_not_exportable (tests/exporters/test_export.py): open(inspect.getfile(...))
• test_transformers_specific_model_import (tests/utils/test_import_structure.py): open(models_path / arch / f"{module}.py")

On Windows with a non-UTF-8 locale (e.g. cp936) these raise UnicodeDecodeError for any of the 91 modeling_*.py sources that contain non-ASCII characters:

tests\exporters\test_export.py:585: in _skip_if_not_exportable
    source_code = f.read()
E   UnicodeDecodeError: 'gbk' codec can't decode byte 0x94 in position 9571: illegal multibyte sequence

tests\utils\test_import_structure.py:107
E   UnicodeDecodeError: 'gbk' codec can't decode byte 0x81 in position 1001: illegal multibyte sequence

The first failure aborts every test_torch_export / ONNX / ExecuTorch variant of the affected model test classes before any export is attempted; the second fails the import-structure consistency check wholesale.

Fix

Pin encoding="utf-8" on both reads — Transformers sources are UTF-8. Same class of fix as #48819.

Verification

Windows 11, cp936 locale, Python 3.14:

• BeitModelTest::test_torch_export_{dynamic,static}: fail without the fix (above), pass with it (2 passed, 10 subtests passed).
• test_transformers_specific_model_import: dozens of subtest failures without the fix, 1 passed + 516 subtests passed with it.

🤖 Generated with Claude Code

[Rocketknight1]

LGTM! Also cc @tarekziade I've seen a couple of these, maybe a Ruff rule to enforce open encodings where using the default might cause locale-dependent behaviour?

[github-actions]

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CI recap

Dashboard: View test results in Grafana
Latest run: 35434920874:2
Result: cancelled | Jobs: 16 | Tests: 190,604 | Failures: 0 | Duration: 17h 37m

[tarekziade]

LGTM! Also cc @tarekziade I've seen a couple of these, maybe a Ruff rule to enforce open encodings where using the default might cause locale-dependent behaviour?

good idea that's PLW1514 I can add it

[HuggingFaceDocBuilderDev]

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[tarekziade]

LGTM! Also cc @tarekziade I've seen a couple of these, maybe a Ruff rule to enforce open encodings where using the default might cause locale-dependent behaviour?

here #48990