Tests: get_accounts(): Improve Assertions

[n7studios]

Summary

Improves assertions to confirm the expected plan and timezone data is included when making calls to get_accounts():
https://developers.kit.com/changelog#july-9-2026-%E2%80%94-accounts

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)