Fix: Form Builder: Submit form to the URL it was displayed on

[n7studios]

Summary

Fixes the Form Builder block submitting to the Post's permalink, instead of the page the form was displayed on.

The form's action was always get_permalink() of the Post containing the block. If the block was displayed elsewhere, such as a blog archive showing full post content, submitting the form took the visitor away from that page to the Post. There, the order of Form Builder blocks can differ, so the validation error from #1212 may not display on the submitted block.

Forms now submit to the URL they were displayed on, excluding the ck_subscriber_id query parameter. A successful submission with no redirect configured also returns the visitor to that URL, rather than the Post's permalink. If the request URI isn't available, the Post's permalink is used, as before.

Testing

New Integration test in BlockFormBuilderMultipleFormsTest:

• testFormActionIsCurrentURL: the form's action is the current URL, with ck_subscriber_id removed and other query parameters kept. Fails without this fix.

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
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• 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)