Document the high-level messaging client REST surface following the
existing per-endpoint-file structure:
- POST/DELETE /messaging/v1/subscriptions (subscribe by content topic)
- POST /messaging/v1/messages (send a MessageEnvelope -> requestId)
- GET /messaging/v1/events/send[/{requestId}] (poll send events)
- GET /messaging/v1/events/received (poll received messages)
Adds the messaging schemas to schemas/apitypes.yaml (MessagingJsonEnvelope,
MessagingSendResponse, SendEventRecord, SendStatus, ReceivedMessageRecord,
list wrappers) and wires the paths + a `messaging` tag into openapi.yaml.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
