Fix ClassVar forward reference detection

[wanxiankai]

Summary

Fixes #1575.

_is_class_var() now unwraps ForwardRef.__forward_arg__ before applying the existing string-prefix checks.
This lets attrs recognize Python 3.14-style forward references such as ForwardRef("ClassVar[str]", is_class=True) as class variables when ClassVar was only imported under TYPE_CHECKING.

Tests cover both the direct _is_class_var() case and the Python 3.14 attrs.define behavior from the issue.

Pull Request Checklist

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  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
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  • New functions/classes have to be added to docs/api.rst by hand.
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[codspeed-hq]

Merging this PR will degrade performance by 61.12%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 2 regressed benchmarks
✅ 15 untouched benchmarks
⏩ 2 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	import attrs	49.2 ms	127 ms	-61.28%
❌	WallTime	import attr	48.7 ms	124.9 ms	-60.96%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing wanxiankai:fix/classvar-forwardref (8328ba7) with main (c27ba27)²}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (ca0f045) during the generation of this report, so c27ba27 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩