Issue #3607820: Reassign escalation only offers assignment audiences (fix cron poison pill)

Frank Mablyrequested to merge
issue/orchestra-3607820:3607820-reassign-timeout-action into 1.x

Filters the reassign timeout action's escalation options to AssignmentInterface, guards onTimeout() against a non-assignment plugin, and rejects one in validation, so a notify-only audience can no longer be selected and fatal on the cron sweep. Adds kernel tests. See #3607820.

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