Creates up to 100 tasks in one project atomically. One project write
check covers the batch; any invalid task rolls the whole batch back
with error 4031 naming the offending payload index, and an
invalid batch size returns 4030. The 1..100 bound is declared as
minItems/maxItems in the schema (pinned to the constant by a test) with
the model check as backstop for non-HTTP callers. Payload positions are
zeroed — positions are always calculated server-side.
