Fix os mock for @actions/core 1.11.x platform module compatibility

@actions/core 1.11.0 added a platform module that calls os.platform()
at import time via __importDefault, which requires a `default` export
on the mocked os module.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: hsbt

__tests__/main.test.ts

@@ -22,6 +22,7 @@ jest.mock('os', () => {
 
   return {
     __esModule: true, // Use it when dealing with esModules
+    default: originalModule,
     ...originalModule,
     homedir: jest.fn()
   }