feat(cli): make /btw command non-blocking with parallel execution

- Add btwItem state management independent from pendingItem
- Add cancelBtw functionality to abort in-flight BTW API calls
- Allow /btw commands to execute concurrently with main responses
- Add isBtwCommand utility function
- Update BtwMessage UI with cleaner styling (remove spinner)
- Add tests for concurrent /btw execution scenarios
- Update layouts to render BTW messages in fixed bottom area

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/test-utils/mockCommandContext.ts

@@ -55,6 +55,10 @@ export const createMockCommandContext = (
       setDebugMessage: vi.fn(),
       pendingItem: null,
       setPendingItem: vi.fn(),
+      btwItem: null,
+      setBtwItem: vi.fn(),
+      cancelBtw: vi.fn(),
+      btwAbortControllerRef: { current: null },
       loadHistory: vi.fn(),
       toggleVimEnabled: vi.fn(),
       extensionsUpdateState: new Map(),