Clarifies the agent availability API documentation so request payloads
use the writable `availability` field, while `availability_status`
remains documented as a read-only response field.
## Closes
Closes#13873
## Why
The backend already supports updating an agent's configured availability
through `availability`, but the Swagger request payloads documented
`availability_status`. That made clients follow a read-only response
field and see successful requests without the intended availability
change.
## What changed
- Replaces `availability_status` with `availability` in agent
create/update request schemas.
- Updates the availability enum to `online`, `busy`, and `offline`.
- Marks response `availability_status` as read-only and explains that it
is derived from configured availability, auto-offline, and presence.
- Regenerates the combined and tag-group Swagger JSON files.
## Validation
- `bundle exec rails swagger:build`
- `bundle exec rspec spec/swagger/openapi_spec.rb`
- `git diff --check`
