Fix pytest collection failure for classes decorated with context managers

[kaxil]

Classes decorated with @conf_vars and other context managers were disappearing during pytest collection, causing tests to be silently skipped. This affected several test classes including TestWorkerStart in the Celery provider tests.

Root cause: ContextDecorator transforms decorated classes into callable wrappers. Since pytest only collects actual type objects as test classes, these wrapped classes are ignored during collection.

Simple reproduction (no Airflow needed):

import contextlib
import inspect

@contextlib.contextmanager
def simple_cm():
    yield

@simple_cm()
class TestExample:
    def test_method(self):
        pass

print(f'Is class? {inspect.isclass(TestExample)}')  # False - pytest won't collect

and then run

pytest test_example.py --collect-only

Airflow reproduction:

Before

> breeze run pytest providers/celery/tests/unit/celery/cli/test_celery_command.py --collect-only -v

collected 6 items

<Dir airflow>
  <Dir providers>
    <Dir celery>
      <Dir tests>
        <Package unit>
          <Package celery>
            <Package cli>
              <Module test_celery_command.py>
                <Class TestRemoteCeleryControlCommands>
                  <Function test_list_celery_workers>
                  <Function test_shutdown_worker>
                  <Function test_shutdown_all_workers>
                  <Function test_add_queue>
                  <Function test_remove_queue>
                <Function test_stale_bundle_cleanup>

After:

> breeze run pytest providers/celery/tests/unit/celery/cli/test_celery_command.py --collect-only -v

collected 14 items

<Dir airflow>
  <Dir providers>
    <Dir celery>
      <Dir tests>
        <Package unit>
          <Package celery>
            <Package cli>
              <Module test_celery_command.py>
                <Class TestCeleryStopCommand>
                  <Function test_if_right_pid_is_read>
                  <Function test_same_pid_file_is_used_in_start_and_stop>
                  <Function test_custom_pid_file_is_used_in_start_and_stop>
                <Class TestWorkerStart>
                  <Function test_worker_started_with_required_arguments>
                <Class TestWorkerFailure>
                  <Function test_worker_failure_gracefull_shutdown>
                <Class TestFlowerCommand>
                  <Function test_run_command>
                  <Function test_run_command_daemon_v_3_below>
                  <Function test_run_command_daemon_v3_above>
                <Class TestRemoteCeleryControlCommands>
                  <Function test_list_celery_workers>
                  <Function test_shutdown_worker>
                  <Function test_shutdown_all_workers>
                  <Function test_add_queue>
                  <Function test_remove_queue>
                <Function test_stale_bundle_cleanup>

Solution:

1. Fixed affected test files by replacing class-level @conf_vars decorators with pytest fixtures
2. Created pytest fixtures to apply configuration changes
3. Used @pytest.mark.usefixtures to apply configuration to test classes
4. Added custom linter to prevent future occurrences and integrated it into pre-commit hooks

Files changed:

• Fixed 3 test files with problematic class decorators
• Added custom linter with pre-commit integration

This ensures pytest properly collects all test classes and prevents similar issues in the future through automated detection.