feat: expose busy_behavior on chat message API (#24054)

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

coderd/exp_chats.go

@@ -1830,6 +1830,20 @@ func (api *API) postChatMessages(rw http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	busyBehavior := chatd.SendMessageBusyBehaviorQueue
+	switch req.BusyBehavior {
+	case codersdk.ChatBusyBehaviorInterrupt:
+		busyBehavior = chatd.SendMessageBusyBehaviorInterrupt
+	case codersdk.ChatBusyBehaviorQueue, "":
+		// Default to queue.
+	default:
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Invalid busy_behavior value.",
+			Detail:  `Must be "queue" or "interrupt".`,
+		})
+		return
+	}
+
 	sendResult, sendErr := api.chatDaemon.SendMessage(
 		ctx,
 		chatd.SendMessageOptions{
@@ -1837,7 +1851,7 @@ func (api *API) postChatMessages(rw http.ResponseWriter, r *http.Request) {
 			CreatedBy:     apiKey.UserID,
 			Content:       contentBlocks,
 			ModelConfigID: req.ModelConfigID,
-			BusyBehavior:  chatd.SendMessageBusyBehaviorQueue,
+			BusyBehavior:  busyBehavior,
 			MCPServerIDs:  req.MCPServerIDs,
 		},
 	)