Domain alias cannot save

Thanks @kergand for creating an issue. Investigating.

Root cause

DomainAliasForm::validateForm() was validating $this->entity->toArray() directly, but never called $this->buildEntity() first. In Drupal's EntityForm lifecycle, validateForm() runs before submitForm(), so the entity still has its default property values — not the submitted form values.

For a new alias, the $redirect property on DomainAlias has no default value, so it is NULL. The config schema defines a Choice constraint allowing only [0, 301, 302], and NULL does not match — hence the validation error "The value you selected is not a valid choice."

A secondary issue: HTML <select> elements always submit string values ("0", "301", "302"), but the schema Choice constraint compares against integers. Even after building the entity from form values, the string-to-integer mismatch would still cause a failure without an explicit cast.

Fix

Two changes in DomainAliasForm::validateForm():

1. Cast the redirect form value to (int) before building the entity, so the integer type matches the schema's Choice constraint.
2. Call $this->buildEntity($form, $form_state) to populate the entity from submitted form values before running validation — instead of validating the stale $this->entity.

Test coverage

Added DomainAliasFormTest — a functional test that submits the alias creation form via the UI for each valid redirect value (0, 301, 302), then edits an existing alias to change its redirect. Verified that the test fails without the fix and passes with it.

@kergand can you confirm that this issue's MR fixes the problem? Thanks.

Thanks. Merged.