[Regression] entity type label_count property no longer enforces its sub-properties

Problem/Motivation

#3396166: Convert entity type discovery to PHP attributes introduced a regression in DX and possibly translatability:

label_count used to be a PluralTranslation, which enforced the keys:

  public function __construct(array $values) {
    if (!isset($values['singular'])) {
      throw new \InvalidArgumentException('Missing "singular" value in the PluralTranslation annotation');
    }
    if (!isset($values['plural'])) {
      throw new \InvalidArgumentException('Missing "plural" value in the PluralTranslation annotation');
    }

Now it's just an array and there's no check of the keys:

    public readonly array $label_count = [],

Steps to reproduce

1. Change the label_count property of an entity type to no longer had one of its child items.
2. Do a rebuild.
3. Note absence of helpful exception

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet