Fix pytest class-scoped fixture deprecation warning in extract_permissions tests

[eladkal]

Running scripts/tests/ci/prek/test_extract_permissions.py surfaces:

=========================================================================================================== warnings summary ============================================================================================================
tests/ci/prek/test_extract_permissions.py::TestExtractAllPermissions::test_extracts_non_empty_result
tests/ci/prek/test_extract_permissions.py::TestRenderRst::test_rst_contains_auto_generated_marker
  /Users/eladkal/Projects/airflow/.venv/lib/python3.12/site-packages/_pytest/fixtures.py:1313: PytestRemovedIn10Warning: Class-scoped fixture defined as instance method is deprecated.
  Instance attributes set in this fixture will NOT be visible to test methods,
  as each test gets a new instance while the fixture runs only once per class.
  Use @classmethod decorator and set attributes on cls instead.
  See https://docs.pytest.org/en/stable/deprecations.html#class-scoped-fixture-as-instance-method
    fixturefunc = resolve_fixture_function(fixturedef, request)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=================================================================================================== 1017 passed, 2 warnings in 10.58s ===================================================================================================

for the class-scoped all_entries fixture in TestExtractAllPermissions
and the rst_content fixture in TestRenderRst. pytest 9.1 deprecates
this pattern because attributes set on self in a class-scoped fixture
aren't visible to test methods — pytest creates a new instance per
test while the fixture runs once per class — and the warning will
become an error in pytest 10.

Both fixtures here just return a computed value and never touch self,
so the fix is mechanical: decorate each with @classmethod and take
cls instead of self, per pytest's documented migration path.

No test behavior changes — all 70 tests in the file still pass with
no warnings.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude (Sonnet 4.5)

Generated-by: Claude (Sonnet 4.5) following the guidelines