pollOutboundMessages

Poll outbound messages for an integration's poll-mode use cases. Takes a lease on the head-of-line batch of the integration's FIFO stream: the returned messages stay invisible to subsequent polls until they are acknowledged or the visibility timeout elapses. POST because taking a lease mutates server state — auto-retrying middleware must not burn leases. One in-flight batch per stream: while a lease is active (or another poll wins the race), the response is an empty batch (messages: []), not an error. Requires the integration:consume grant.