[DeadCode] Skip static/$this union @return in RemoveUselessUnionReturnDocblockRector

[TomasVotruba]

static and $this denote the runtime subclass, which a native return type like ?SomeInterface cannot express. Removing such a @return static|null docblock loses information, so it must be kept.

Reported on a Mautic trait:

/**
 * @return static|null
 */
public function getVariantParent(): ?VariantEntityInterface
{
    return $this->variantParent;
}

When static cannot resolve to a concrete class (e.g. a trait or free function in the analyzed reflection scope), it degrades to mixed, which is broader than the native type and the rule wrongly stripped the docblock.

Fix: skip the union @return when it contains static or $this. Added skip fixtures for both the trait and the degraded (free-function) case.