Forms: form validation does not work when JavaScript is disabled

### Impacted plugin

Jetpack

### Quick summary

The Form block (and possibly shortcode) usually performs JavaScript form validation. See the [accessible-form.js](https://github.com/Automattic/jetpack/blob/trunk/projects/packages/forms/src/contact-form/js/accessible-form.js) file.

There's also a lot of PHP code for form validation, which is (I assume) for when JavaScript is disabled. See the `Contact_Form_Field->validate` method:
https://github.com/Automattic/jetpack/blob/7a3559d2f7bd0cd8915902a4d0f315167793e5cd/projects/packages/forms/src/contact-form/class-contact-form-field.php#L204-L266

This PHP validation doesn't seem to work, at least on a block theme. Submitting a form that has validation errors sends the user to a 404 page.

If this needs to be supported, then it looks like something to be fixed. Alternatively, the code could be removed if form validation with JS disabled isn't a feature that needs to be kept.

### Steps to reproduce

1. Create a form with at least one required field in a post and view the post
2. In the browser dev tools, disable JS then reload the page to make sure that take effect
3. Submit the form

Expected: The form shows validation errors
Actual: A 404 page is shown. This seems to be because `?page=1` is appended to the URL.

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Minor

### What other impact(s) does this issue have?

No revenue impact

### If a workaround is available, please outline it here.

_No response_

### Platform (Simple and/or Atomic)

Self-hosted

Reproduce the form submission with JavaScript disabled, then inspect projects/packages/forms/src/contact-form/class-contact-form-field.php, especially Contact_Form_Field->validate, alongside projects/packages/forms/src/contact-form/js/accessible-form.js. Trace why the validation response appends ?page=1 and reaches a 404; done means invalid forms show their validation errors without JavaScript, or the repository documents that this behavior is intentionally unsupported.

Written by the indexing model from the issue text.