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

codersdk/chats.go

@@ -1346,20 +1346,6 @@ func (c *Client) GetChatGitChanges(ctx context.Context, chatID uuid.UUID) ([]Cha
 	return changes, json.NewDecoder(res.Body).Decode(&changes)
 }
 
-// GetChatDiffStatus returns cached GitHub pull request diff status for a chat.
-func (c *Client) GetChatDiffStatus(ctx context.Context, chatID uuid.UUID) (ChatDiffStatus, error) {
-	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/experimental/chats/%s/diff-status", chatID), nil)
-	if err != nil {
-		return ChatDiffStatus{}, err
-	}
-	defer res.Body.Close()
-	if res.StatusCode != http.StatusOK {
-		return ChatDiffStatus{}, ReadBodyAsError(res)
-	}
-	var status ChatDiffStatus
-	return status, json.NewDecoder(res.Body).Decode(&status)
-}
-
 // GetChatDiffContents returns resolved diff contents for a chat.
 func (c *Client) GetChatDiffContents(ctx context.Context, chatID uuid.UUID) (ChatDiffContents, error) {
 	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/experimental/chats/%s/diff", chatID), nil)