Show content validation errors when Saving Ajaxy

[bobvandevijver]

Resubmit of #3769.

[Copilot]

Pull request overview

This PR fixes the “ajaxy save” path so that content-validation failures are returned as JSON (rather than HTML) and can be displayed in the editor UI without a full page reload, matching the classic (non-ajax) UX for validation feedback.

Changes:

• Add a dedicated JSON (HTTP 422) response for content-validation violations on XHR saves.
• Update the ajaxy-save JS handler to safely render validator messages (text-only) and show inline alert blocks above the edit form.
• Add a PHPUnit test covering the controller’s ajaxy validation-error JSON contract.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/php/Controller/Backend/ContentEditControllerSaveTest.php	Adds a controller unit test for ajaxy validation-error JSON responses.
src/Controller/Backend/ContentEditController.php	Returns validator violations as JSON (422) for XHR saves via a new helper.
assets/js/app/ajax-save.js	Displays validation errors inline and sanitizes toast body by using .text().

Suppressed comments (2)

tests/php/Controller/Backend/ContentEditControllerSaveTest.php:100

• Setting the protected $config property via ReflectionProperty::setValue() requires setAccessible(true) (as done in other tests like tests/php/Twig/TokenParserTestCase.php). Without it, this assignment can fail and leave $this->config uninitialized, breaking the test.

        (new ReflectionProperty(TwigAwareController::class, 'config'))->setValue($controller, $this->configWithValidatorEnabled());

assets/js/app/ajax-save.js:139

• Code style in this file consistently uses a space between function and the parameter list (e.g. function (toastEl) above). The new callback uses function(error), which is inconsistent and may trip linting.

                    let messages = response.errors.map(function (error) {

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