Implement test for app-b Bearer token usage

[drQedwards]

This pull request adds a new test to ensure that when multiple apps are configured with different authentication methods, the API client correctly uses the Bearer token for an app that only has a Bearer token, even if the default app has OAuth2 credentials.

Authentication handling improvements:

• Added a new test, TestTC5_3_ApiClientUsesAppBBearerNotDefaultOAuth2, to verify that when switching from a default app with OAuth2 credentials to another app (app-b) with only a Bearer token, the API client uses the correct Bearer token for authentication instead of falling back to the default app's OAuth2 token.
• Introduced a helper function futureExpiry to generate a future expiration timestamp for OAuth2 tokens in tests.

Test utilities:

• Added the time package import to support token expiration handling in tests.Add test for ApiClient using app-b's Bearer token instead of app-a's OAuth2.