When a MorphTo relation's docblock uses a bare intersection type in the
generic parameter (e.g. `MorphTo<BaseModel&CanBeAssigned, $this>`), the
generated property type incorrectly becomes `BaseModel&CanBeAssigned|null`
instead of the correct `(BaseModel&CanBeAssigned)|null`.
This adds `wrapIntersectionType()` which ensures bare intersection types
are wrapped in parentheses before `|null` is appended, producing valid
DNF type syntax. Applied in both `setProperty()` and `applyNullability()`.
