docs(integration-toolkit): document the delivered payload on poll MSG_ENQUEUED

[karlrank]

Summary

Follow-up to #159. Documents what integration-toolkit-api !261 shipped: a poll message's MSG_ENQUEUED monitoring event now shows the payload a transform produced.

• Pollable Outbound: new section "What a mapped message looks like in monitoring", linked from the MSG_ENQUEUED row.
  • A mapped message's detail is { message_id, event_name, mapping_version, payload }, where payload is exactly what the poll API delivers.
  • Above 48 KiB the payload is replaced by payload_omitted: { reason: "too_large", size_bytes, limit_bytes: 49152 }. The page explains in one line why the limit exists: a monitoring page returns up to 100 events inline.
  • Raw and failed messages keep { message_id, event_name }. A raw message stores no second copy, because the trigger event already shows what was delivered.
  • A caution note says that mapped payloads, and so customer data, are copied into monitoring and that reading them requires integration:view.
• Investigating events and Configuration → Permissions: the monitoring events list (POST /v2/integrations/{id}/monitoring/events) requires integration:view on the integration and returns 403 without it.
• Monitoring Overview: the detail field description mentions the delivered payload.
• Monitoring Codes: regenerated from the merged monitoring-codes.json snapshot. Only the MSG_ENQUEUED description changes.

Verification

• I checked every claim against the merged backend on main: delivered-payload-detail.ts, handle-event-catalog.ts, monitoring-handlers-v2.ts, and openapi.yml 1.38.2.
• npm run build passes with no broken links or anchors.

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