refactor: remove /diff-status endpoint, include diff_status in chat payload (#23082)

The `/chats/{chat}/diff-status` endpoint was redundant because:
- The `Chat` type already has a `DiffStatus` field
- Listing chats already resolves and returns `diff_status`
- The `getChat` endpoint was the only one not resolving it (passing
`nil`)

## Changes

**Backend:**
- `getChat` now calls `resolveChatDiffStatus` and includes the result in
the response
- Removed `getChatDiffStatus` handler, route (`GET /diff-status`), and
SDK method
- Tests updated to use `GetChat` instead of `GetChatDiffStatus`

**Frontend:**
- `AgentDetail.tsx`: uses `chatQuery.data?.diff_status` instead of
separate query
- `RemoteDiffPanel.tsx`: accepts `diffStatus` as a prop instead of
fetching internally
- `AgentsPage.tsx`: `diff_status_change` events now invalidate the chat
query
- Removed `chatDiffStatus` query, `chatDiffStatusKey`, and
`getChatDiffStatus` API method

coderd/coderd.go

@@ -1187,7 +1187,6 @@ func New(options *Options) *API {
 				r.Patch("/messages/{message}", api.patchChatMessage)
 				r.Get("/stream", api.streamChat)
 				r.Post("/interrupt", api.interruptChat)
-				r.Get("/diff-status", api.getChatDiffStatus)
 				r.Get("/diff", api.getChatDiffContents)
 				r.Route("/queue/{queuedMessage}", func(r chi.Router) {
 					r.Delete("/", api.deleteChatQueuedMessage)