Add warning when bundle path isn't accessible to impersonated user

[Dev-iL]

- This PR was created with the help of AI

Related: #60270, #60387

Context

When using Git DAG bundles with user impersonation (run_as_user), tasks fail due to permission conflicts. The bundle is cloned by the main airflow user, but the impersonated user needs to access the repository files during task execution.

This PR provides early warning to help users diagnose these issues before they cause task failures.

Changes

_verify_bundle_access(bundle_instance, log)

New helper function in task_runner.py that:

• Uses os.access() to verify actual read/execute permissions (works with Unix permissions, ACLs, SELinux, etc.)
• Raises AirflowException with a clear error message if the bundle is not accessible
• Includes a link to documentation for troubleshooting

The helper is called in parse() after bundle_instance.initialize(), which means it runs after impersonation occurs. This ensures the check reflects the impersonated user's actual access rights.

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[github-actions]

Backport failed to create: v3-1-test. View the failure log Run details

Status	Branch	Result
❌	v3-1-test

You can attempt to backport this manually by running:

cherry_picker 0e0fceb v3-1-test

This should apply the commit to the v3-1-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue