fix(extension): make tool cancellation cooperative (#89)

Tool cancellation is now cooperative end-to-end: the queue stays busy until compensation completes, cancellation is checked cooperatively across the pipeline, the fallback window is compensated too, and cleanup_failed is surfaced explicitly. Includes a gated delayed-compensation E2E test.

apps/extension/src/transport/types.ts

@@ -32,7 +32,8 @@ export type RpcErrorReason =
   | "tab_not_active"
   | "restricted_tab_url"
   | "borrow_conflict"
-  | "screenshot_capture_failed";
+  | "screenshot_capture_failed"
+  | "cleanup_failed";
 
 export interface RpcErrorData {
   reason?: RpcErrorReason;