The backend (`chatd.go`) already fully implements both `"queue"` and
`"interrupt"` busy behaviors for `SendMessage`, and the `message_agent`
subagent tool already leverages both internally. However the HTTP API
hardcoded `"queue"` and the SDK had no way for callers to request
interrupt-on-send.
This adds a `ChatBusyBehavior` enum type to the SDK and an optional
`busy_behavior` field on `CreateChatMessageRequest`. The HTTP handler
validates the field and passes it through to `chatd.SendMessage`.
Default remains `"queue"` for full backward compatibility.
<details><summary>Implementation notes</summary>
- `codersdk/chats.go`: New `ChatBusyBehavior` type with
`ChatBusyBehaviorQueue` and `ChatBusyBehaviorInterrupt` constants. Added
`BusyBehavior` field to `CreateChatMessageRequest` with `enums` tag for
codegen.
- `coderd/exp_chats.go`: `postChatMessages` now reads
`req.BusyBehavior`, maps SDK constants to
`chatd.SendMessageBusyBehavior*`, returns 400 on invalid values.
- `site/src/api/typesGenerated.ts`: Auto-generated via `make gen`.
- No frontend behavior changes — the field is available but unused by
the UI.
</details>
> [!NOTE]
> Generated by Coder Agents
Kyle Carberry
648787e739