test(gen2-migration): add test for post confirmation trigger in store-locator app

[sai-ray]

Description of changes

This PR adds a test that validates the PostConfirmation Cognito trigger for store-locator. A user who signs up and gets confirmed should land in the storeLocatorAdmin group via the migrated Lambda. The test runs at every test:gen1 and test:gen2 phase, exercising the trigger on both the Gen1 pool and the Gen2 post-refactor pool.

The trigger only fires on the real SignUp + AdminConfirmSignUp flow, which requires AllowAdminCreateUserOnly: false on the user pool. The test detects the current value, flips to false for the test run if needed, and restores the original value in afterAll.

Cognito's UpdateUserPool resets any omitted field to its default, so the test follows the AWS-documented pattern of DescribeUserPool → copy all safe fields → override just AdminCreateUserConfig.AllowAdminCreateUserOnly → UpdateUserPool to avoid clobbering unrelated pool settings.

Issue #, if available

Description of how you validated changes

• Full e2e run on store-locator passed end-to-end: 8 invocations of tests/auth.test.ts across all test:gen1 / test:gen2 phases (pre/post sandbox and pre/post refactor), plus the existing shared-auth.test.ts at the end. No regressions to other tests.

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.