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

codersdk/chats.go

@@ -369,11 +369,27 @@ type UpdateChatRequest struct {
 	Labels   *map[string]string `json:"labels,omitempty"`
 }
 
+// ChatBusyBehavior controls what happens when a user sends a message
+// while the chat is already processing.
+type ChatBusyBehavior string
+
+const (
+	// ChatBusyBehaviorQueue queues the message for processing after
+	// the current run finishes.
+	ChatBusyBehaviorQueue ChatBusyBehavior = "queue"
+	// ChatBusyBehaviorInterrupt queues the message and interrupts
+	// the active run. The partial assistant response is persisted
+	// before the queued message is promoted, preserving correct
+	// conversation order.
+	ChatBusyBehaviorInterrupt ChatBusyBehavior = "interrupt"
+)
+
 // CreateChatMessageRequest is the request to add a message to a chat.
 type CreateChatMessageRequest struct {
-	Content       []ChatInputPart `json:"content"`
-	ModelConfigID *uuid.UUID      `json:"model_config_id,omitempty" format:"uuid"`
-	MCPServerIDs  *[]uuid.UUID    `json:"mcp_server_ids,omitempty" format:"uuid"`
+	Content       []ChatInputPart  `json:"content"`
+	ModelConfigID *uuid.UUID       `json:"model_config_id,omitempty" format:"uuid"`
+	MCPServerIDs  *[]uuid.UUID     `json:"mcp_server_ids,omitempty" format:"uuid"`
+	BusyBehavior  ChatBusyBehavior `json:"busy_behavior,omitempty" enums:"queue,interrupt"`
 }
 
 // EditChatMessageRequest is the request to edit a user message in a chat.