Accept FormFactoryInterface in polymorphic collection types

[javihgil]

Problem

PolymorphicCollectionType and DoctrinePolymorphicCollectionType currently require the concrete FormFactory class instead of FormFactoryInterface.

Current behavior

Passing a valid Symfony form factory implementation that only satisfies FormFactoryInterface is rejected.

Expected behavior

The component should accept FormFactoryInterface, which is the public Symfony contract and the safer dependency for integrations and testing.

Why it matters

This makes the component harder to reuse and harder to test in custom integrations, even when the provided factory is fully compatible.