api_endpoint channels convert to Agent API channels
The api_endpoint channel type is removed. At startup, each existing
api_endpoint channel converts to an Agent API (api) channel: its keys are
imported with the same hash, its sessions are retagged, and its settings map to
the new channel.
Clients must move to the Agent API routes under /v1/channels/{channel_id}.
Old request shapes get 400 api_endpoint_retired pointing at the Agent API
docs, and /v1/apps/{app}/api/{chan} routes return 410.
See Call your agent from code, Agent API and #4527.