fix: prompt caching, conversation length management and token usage display

tests/renderer/a2ui/surfaceActionWiring.test.tsx

@@ -81,6 +81,7 @@ function setupChatApi() {
     onToolResult: vi.fn(() => vi.fn()),
     onTitleUpdated: vi.fn(() => vi.fn()),
     onA2UIMessage: vi.fn(() => vi.fn()),
+    onTokenUsage: vi.fn(() => vi.fn()),
     dispatchA2UIAction: vi.fn(),
   } as never;
 }