Wire ReflectionEnum into existing infrastructure

[Copilot]

ReflectionClassConstant had no integration with the ReflectionEnum stack: enum case constants couldn't be looked up by name via the engine, getDeclaringClass() returned a plain ReflectionClass instead of ReflectionEnum, and there was no way to obtain a properly-typed ReflectionEnumUnitCase/ReflectionEnumBackedCase from a constant.

Changes

• ReflectionEngine::parseClassConstant() — now searches EnumCase nodes in addition to ClassConst nodes, so new ReflectionClassConstant('MyEnum', 'CaseName') no longer throws InvalidArgumentException.

• ReflectionClassConstant::getDeclaringClass() — returns ReflectionEnum when isEnumCase() is true, matching native PHP where \ReflectionClassConstant::getDeclaringClass() returns \ReflectionEnum for enum case constants.

• ReflectionClassConstant::toEnumCase() — new method converting an enum-case constant to ReflectionEnumUnitCase or ReflectionEnumBackedCase (delegates to ReflectionEnum::getCase()); throws ReflectionException if called on a non-enum-case constant.

$parsedEnum     = $fileNamespace->getEnums()[BackedPhp81EnumHTTPMethods::class];
$parsedConstant = $parsedEnum->getReflectionConstant('GET'); // ReflectionClassConstant

$parsedConstant->getDeclaringClass(); // ReflectionEnum (was ReflectionClass)
$parsedConstant->toEnumCase();        // ReflectionEnumBackedCase, getBackingValue() => 'get'