Do not let placeholder members of intersection members override declared members

[ondrejmirtes]

MixedType - and so T of mixed - answers hasMethod(), hasProperty() and hasConstant() with yes for every name and describes the member with a Dummy*Reflection placeholder declared on stdClass. IntersectionType merged those placeholders with the members declared by the other intersected types, so on T&Foo:

• methods: since 857a5f4 IntersectionTypeMethodReflection prefers a variadic signature, so the placeholder (a variadic TrivialParametersAcceptor) always won - getDeclaringClass() returned stdClass and $x->bar(...) lost its parameters
• properties: the placeholder provided the declaring class when it came first - errors talked about stdClass::$prop
• constants: getConstant() returned the placeholder when the mixed member came first

Placeholders are now used only when no member declares the member.

The method part fixes the shopware/shopware integration test, red since 857a5f4 (2026-09-17): phpstan-symfony's RequestTypeSpecifyingExtension calls $methodReflection->getDeclaringClass()->getNativeMethod('getSession') and crashed with "Method getSession() was not found in reflection of class stdClass" on

static::callback(function (Request $request) {
    return $request->hasSession()
        && $request->getSession() instanceof Session;
})

where the closure parameter is CallbackInput&Request while PHPUnit's callback() template is being inferred.

On 2.3.x, IntersectionType is shadowed by turbo-ext/src/IntersectionType.cpp (unresolvedPrototype(), getConstant()), so the merge-up needs the same change there.

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