Create a new test case for a product. The draftSnapshot is stored verbatim so the placeholder-then-fill pattern keeps working. The 200 response includes a non-blocking validationWarnings: DraftSnapshotValidationWarning[] array — per-field validation issues against the TransportForProducts shape. An empty array means the snapshot would validate cleanly at POST /test-cases/run time. A wrapped envelope with transport: null or a bare TransportForProducts (no { transport, shapeId } wrapper) still 400s — validationWarnings[] is only populated for an otherwise-acceptable but underspecified transport.