The turn- and api-scoped branches each repeated the same
task/session/thread fallback ladder with only the infix differing. Extract
the shared prefix into _scope_prefix so a future scope dimension touches one
ladder instead of three. The legacy branch still returns a bare task_id (not
the task: prefix) for backward compatibility, so it stays separate.
Output key strings are unchanged; a new test pins them across every
task/session/turn/api combination since the keys are matched across hooks
and any drift would silently break trace finalization.
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