Add tests for enableExpoHermes app.json updates

[ConnorQi01]

Summary

• Add unit coverage for enableExpoHermes app.json manipulation.
• Keep existing jsEngine replacements scoped to the selected platform section.
• Create missing Android/iOS app.json sections before setting jsEngine.

Test Plan

• npm run build
• npm test

At least one added regression test fails without the non-test code change because selecting Android can otherwise update the first jsEngine entry from another platform.

Closes #2738