validateSyncMutationBatch
validateSyncMutationBatch(
mutations):SyncBatchValidationOutcome
Up-front validation shared by both batch transports (HTTP and socket): reject an
oversized batch or one with intra-batch duplicate mutationIds before anything is
applied. On failure, returns a single-element results array carrying a
validation nack for the offending mutation (or an empty-batch guard) — mirroring
the shape callers expect from applySyncMutationBatch, but produced without
touching the idempotency ledger since nothing was attempted.