Fix: Form Builder: Display validation error on the submitted form only

[n7studios]

Summary

Fixes the Form Builder block displaying a validation error on every Form Builder block on a Page, and jumping to the first block, when a visitor submits a form further down the Page.

The submission error was stored once per request, and every Form Builder block on the Page added the error notice and autofocus attribute, so the browser focused the first block. This was reported by a creator with two Form Builder blocks on their homepage: submitting the second block with an invalid email address jumped back to the first block's error.

Each block now outputs its index on the Page as a hidden convertkit[block_index] field. The error notice, aria-invalid and autofocus are only added to the block that was submitted, so the visitor is taken to that block and its error. If no index is submitted (e.g. a page cached before this update), the error is displayed on all blocks, as before.

Testing

New Integration tests in BlockFormBuilderMultipleFormsTest:

• testBlockIndexHiddenField: each block outputs its index. Fails without this fix.
• testErrorDisplayedOnSubmittedBlockOnly: the error notice, aria-invalid and autofocus are only added to the submitted block. Fails without this fix.
• testErrorDisplayedOnAllBlocksWhenNoBlockIndex: the existing behaviour is kept when no index is submitted.

New E2E test, PageBlockFormBuilderCest::testFormBuilderBlockWithInvalidEmailAddressInSecondBlock:

• Adds two Form Builder blocks to a Page and submits the second with an invalid email address.
• Confirms one error notice is displayed, in the second block.
• Confirms the second block's email field is focused.

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)