The kanban test's hook leg used to rewrite the pre-existing attachment, which
the created-paths snapshot already ignores, so the kanban exclusion was never
reached on that path. It now creates a NEW kanban file via the call and
asserts it survives session end. Without the kanban entry in
_NEVER_TRACK_TOP_LEVEL that file gets tracked and deleted.
Also moves the function-local `import os` to module level and replaces
iter(range(1, 10**6)) with itertools.count(1) for call IDs.
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