Summary

Fixes:

• testCreateGetUpdateAndDeleteSequenceEmail, by setting the sequence email as published on creation, as the API won't reflect the specified email_template_id otherwise: https://wpzinc.slack.com/archives/C02KFR6N1GF/p1781001620443109
• testCreateSubscriberWithInvalidCustomFields, as the API now returns a warnings array instead of a 4xx error: https://developers.kit.com/api-reference/subscribers/create-a-subscriber

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)