saveIfNotExistsAndGet

Same LWT write path as saveIfNotExists, but returns the entity actually persisted — with any @GeneratedUuid/@CreatedAt/@UpdatedAt values injectTimestamps computed before the write — instead of discarding that copy behind a bare Boolean the way saveIfNotExists does. Returns null when the IF NOT EXISTS check did not apply (a row already existed under this key) — the same "nothing was written" signal saveIfNotExists reports as false. See GH #138: same root-cause shape as GH #132 (save(), fixed by saveAndGet()) and GH #136 (update(), fixed by updateAndGet()) — a third instance of the same gap in this file.