Moved duplicated ::toArray() methods into a shared trait

Problem/Motivation

Eight backed enums under src/Enum/ share an identical toArray() implementation: iterate self::cases(), map each case through toGraphQlEnumValue(), and return an associative array keyed by the GraphQL enum name with the backing value. The duplication is copy-pasted across WebformElementDescriptionDisplay, WebformElementDisplayOn, WebformElementHelpDisplay, WebformElementTitleDisplay, WebformFormUnavailableReason, WebformMessageType, WebformSubmissionConfirmationType, and WebformWeekday. PHP 8.2+ allows traits on enums, so this can be consolidated without changing the public API.

Proposed resolution

Add a trait, e.g. GraphQlEnumValuesTrait, in src/Enum/ containing the shared static toArray():

public static function toArray(): array {
  $cases = [];
  foreach (self::cases() as $case) {
    $cases[$case->toGraphQlEnumValue()] = $case->value;
  }
  return $cases;
}

Each affected enum adds use GraphQlEnumValuesTrait; and removes its local toArray() method. Each enum keeps its own toGraphQlEnumValue() implementation — naming is not uniform enough to share (explicit match expressions vs. CaseConverter on backing values in enums like WebformElementTrigger, which does not use toArray() today).

Optionally add a small interface, e.g. GraphQlBackedEnumInterface, declaring toGraphQlEnumValue(): string, implemented by enums that use the trait. This documents the contract the trait relies on but is not strictly required.

No changes to callers (WebformSchemaBuilder, etc.) — SomeEnum::toArray() remains the entry point.

Remaining tasks

• Add GraphQlEnumValuesTrait (and optionally GraphQlBackedEnumInterface).
• Update the eight enums listed above to use the trait and drop duplicated toArray() bodies.
• Confirm existing coverage still passes (e.g. WebformElementDisplayOnTest::testToArray()).
• Optionally add a focused unit test on one enum or the trait to guard against regressions.

Introduced terminology

GraphQlEnumValuesTrait — shared trait providing toArray() for backed enums that expose GraphQL enum values via toGraphQlEnumValue().