feat: move chat messages to dedicated /chats/{id}/messages endpoint (#23021)

## Summary

Moves the messages response out of `GET /chats/{id}` and into a
dedicated `GET /chats/{id}/messages` endpoint.

### Backend
- `GET /chats/{id}` now returns just the `Chat` object (no messages)
- `GET /chats/{id}/messages` is a new endpoint returning
`ChatMessagesResponse` with `messages` and `queued_messages`
- Added `ChatMessagesResponse` SDK type and `GetChatMessages` client
method

### Frontend
- `getChat()` API method returns `Chat` instead of `ChatWithMessages`
- Added `getChatMessages()` API method for the new endpoint
- Split `chatQuery` into two: `chatQuery` (metadata) and
`chatMessagesQuery` (messages)
- Updated all cache mutations, optimistic updates, and websocket
handlers
- Updated tests and stories

### Files changed
| File | Change |
|---|---|
| `coderd/coderd.go` | Register `GET /messages` route |
| `coderd/chats.go` | Simplify `getChat`, add `getChatMessages` handler
|
| `codersdk/chats.go` | New type + method, update `GetChat` return |
| `site/src/api/api.ts` | New method, update `getChat` |
| `site/src/api/queries/chats.ts` | New query, update cache mutations |
| `site/src/pages/AgentsPage/AgentDetail.tsx` | Use separate queries |
| `site/src/pages/AgentsPage/AgentDetail/ChatContext.ts` | Update types
and cache writes |
| `site/src/pages/AgentsPage/AgentsPage.tsx` | Update websocket cache
handler |

codersdk/chats.go

@@ -167,9 +167,8 @@ type UploadChatFileResponse struct {
 	ID uuid.UUID `json:"id" format:"uuid"`
 }
 
-// ChatWithMessages is a chat along with its messages.
-type ChatWithMessages struct {
-	Chat           Chat                `json:"chat"`
+// ChatMessagesResponse contains the messages and queued messages for a chat.
+type ChatMessagesResponse struct {
 	Messages       []ChatMessage       `json:"messages"`
 	QueuedMessages []ChatQueuedMessage `json:"queued_messages"`
 }
@@ -980,20 +979,34 @@ func (c *Client) StreamChat(ctx context.Context, chatID uuid.UUID, opts *StreamC
 	}), nil
 }
 
-// GetChat returns a chat by ID, including its messages.
-func (c *Client) GetChat(ctx context.Context, chatID uuid.UUID) (ChatWithMessages, error) {
+// GetChat returns a chat by ID.
+func (c *Client) GetChat(ctx context.Context, chatID uuid.UUID) (Chat, error) {
 	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/experimental/chats/%s", chatID), nil)
 	if err != nil {
-		return ChatWithMessages{}, err
+		return Chat{}, err
 	}
 	defer res.Body.Close()
 	if res.StatusCode != http.StatusOK {
-		return ChatWithMessages{}, ReadBodyAsError(res)
+		return Chat{}, ReadBodyAsError(res)
 	}
-	var chat ChatWithMessages
+	var chat Chat
 	return chat, json.NewDecoder(res.Body).Decode(&chat)
 }
 
+// GetChatMessages returns the messages and queued messages for a chat.
+func (c *Client) GetChatMessages(ctx context.Context, chatID uuid.UUID) (ChatMessagesResponse, error) {
+	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/experimental/chats/%s/messages", chatID), nil)
+	if err != nil {
+		return ChatMessagesResponse{}, err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return ChatMessagesResponse{}, ReadBodyAsError(res)
+	}
+	var resp ChatMessagesResponse
+	return resp, json.NewDecoder(res.Body).Decode(&resp)
+}
+
 func (c *Client) ArchiveChat(ctx context.Context, chatID uuid.UUID) error {
 	res, err := c.Request(ctx, http.MethodPost, fmt.Sprintf("/api/experimental/chats/%s/archive", chatID), nil)
 	if err != nil {