[make:entity] Generate a strict setter for non-nullable unidirectional ManyToOne relations

[Amoifr]

Summary

Concrete design proposal for #198, which reports that generated ManyToOne setters accept null even when the relation is non-nullable (setCategory(?Category $category) for a nullable: false association).

The discussion stalled on a real constraint: the setter must stay nullable for bidirectional relations, because the collection remove*() generated on the inverse side sets the owning side back to null ($item->setOwner(null), orphanRemoval) — making every ManyToOne setter strict would break that generated code.

This PR tightens only the provably-safe subset: a non-nullable, unidirectional ManyToOne, which has no such remove*() null path:

public function setCategory(Category $category): static  // was: ?Category

Bidirectional and explicitly-nullable relations are unchanged. This mirrors what already happens for non-nullable OneToOne setters ($relation->isNullable()), just extended to the ManyToOne case that is provably safe.

Scope / open questions

• Deliberately setter-only (the original report). The backing property stays ?Category $category = null — consistent with the current OneToOne behavior, and it avoids uninitialized-typed-property pitfalls with Forms.
• Also making the property non-nullable (raised again recently re: phpstan-doctrine doctrine.associationType) is a bigger discussion — it reopens @javiereguiluz's Forms concern about uninitialized properties — so it's left out here. Happy to follow up if you'd like to go there.

No existing fixtures change (no current test case is non-nullable + unidirectional); a new many_to_one_not_nullable_no_inverse case + fixture covers it.