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

• I acknowledge this project's AI policy.
• This pull request is not from my main branch.
  • Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.
• There's tests for all new and changed code.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file typing-examples/baseline.py or, if necessary, typing-examples/mypy.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• The documentation has been updated.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changes to the signatures of @attr.s() and @attrs.define() have to be added by hand too.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
    The next version is the second number in the current release + 1.
    The first number represents the current year.
    So if the current version on PyPI is 26.2.0, the next version is gonna be 26.3.0.
    If the next version is the first in the new year, it'll be 27.1.0.
  • Documentation in .rst and .md files is written using semantic newlines.
• Changes have news fragments in changelog.d.