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Description

Fixes #10906

link_error may hold a single signature or a list, and every other reader wraps it with maybe_list(). Backend.chord_error_from_stack() and Backend._handle_group_chord_error() iterated the option directly. A chord body with one errback, such as tsum.s().set(link_error=eb.s()), therefore had the signature's keys iterated. The errback never ran, and one bogus task per key ('task', 'args', 'kwargs', ...) was published, showing up on the worker as "Received unregistered task of type 'task'". For a group body this happened once per body task.

Changes:

• celery/backends/base.py: both sites now pass maybe_list(<callback>.options.get("link_error")) or [], the same idiom used in canvas.py and app/task.py.
• t/unit/backends/test_base.py: test_chord_error_from_stack_accepts_a_single_errback and test_handle_group_chord_error_accepts_a_single_errback, each parametrized over a single signature and a list. The single-signature cases fail on main and pass with this change.

Testing:

• pytest t/unit/backends t/unit/tasks: 1953 passed, 49 skipped, 1 xfailed.
• flake8, isort, codespell, pyupgrade and yesqa, at the versions in .pre-commit-config.yaml, are clean on the changed files.
• The reproduction script from the issue, run on a real worker:

# main
single link_error -> chord state FAILURE, errback called 0 time(s)
list   link_error -> chord state FAILURE, errback called 1 time(s)

# this branch
single link_error -> chord state FAILURE, errback called 1 time(s)
list   link_error -> chord state FAILURE, errback called 1 time(s)

No unregistered-task errors are logged on this branch.