s3tables: keep the fence on a declare of an existing index
Deciding the existing-index exception before the handler ran left a
window: an index deleted between that lookup and the handler's own check
would be recreated, unfenced, by a caller without s3vectors:CreateIndex.
Drop the up-front exception, so every CreateTable runs fenced and a
fresh create is refused at its first write. The engine's declare still
passes because the idempotent path writes nothing; for a build whose
idempotent path re-applies state already in place, the fence answers an
UpdateEntry that leaves the entry exactly as stored without writing.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
