Delete a sequence (?force=true, ?dryRun=true)

Soft-deletes one sequence. Without ?force=true, a sequence with a sent footprint is guarded — 409 SEQUENCE_HAS_SENT_MESSAGES; ?force=true (forceDeletionOfSentMessages) extends the delete to already-sent messages. ?dryRun=true reports the would-be effect ({ dryRun, sequenceId, status, force }) with no writes. Replaces the deprecated POST /sequences/{sequenceId}/delete spelling.