feat(FN-1761): fix lint errors in executor.test.ts and restart.integration.test.ts

- executor.test.ts: remove unused imports (Column, StuckTaskDetector),
  replace Function type with EventListener, add MockTaskStore interface
- restart.integration.test.ts: replace require() with ESM import,
  replace Function types with proper function signatures
- All tests pass

packages/engine/src/plugin-runner.ts

@@ -502,7 +502,7 @@ export class PluginRunner {
         this.hookTimeoutMs,
         `Hook ${hookName} timed out`,
       );
-    } catch (_err) {
+    } catch {
       // Error already logged by invokeHook
     }
   }