chore(rtn-web-browser): add initial unit tests

[ahmedhamouda78]

Description of changes

This PR adds initial unit tests for the rtn-web-browser package. The changes include:

• Added basic unit tests for the openAuthSessionAsync function
• Updated Jest configuration for more reasonable initial test coverage
• Updated package.json test script

Issue #, if available

Issue: #14459
PR: #14523

Description of how you validated changes

• Added unit tests
• Tests validate Chromebook detection logic and basic openAuthSessionAsync behavior
• Verified test coverage meets package requirements
• All tests pass locally

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

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