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 |

coderd/chats.go

@@ -360,6 +360,15 @@ func (api *API) listChatModels(rw http.ResponseWriter, r *http.Request) {
 //
 //nolint:revive // HTTP handler writes to ResponseWriter.
 func (api *API) getChat(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	chat := httpmw.ChatParam(r)
+	httpapi.Write(ctx, rw, http.StatusOK, convertChat(chat, nil))
+}
+
+// EXPERIMENTAL: this endpoint is experimental and is subject to change.
+//
+//nolint:revive // HTTP handler writes to ResponseWriter.
+func (api *API) getChatMessages(rw http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 	chat := httpmw.ChatParam(r)
 	chatID := chat.ID
@@ -385,8 +394,7 @@ func (api *API) getChat(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	httpapi.Write(ctx, rw, http.StatusOK, codersdk.ChatWithMessages{
-		Chat:           convertChat(chat, nil),
+	httpapi.Write(ctx, rw, http.StatusOK, codersdk.ChatMessagesResponse{
 		Messages:       convertChatMessages(messages),
 		QueuedMessages: convertChatQueuedMessages(queuedMessages),
 	})