Add PHPUnit coverage for polymorphic collection behavior and Doctrine variant

[javihgil]

Problem

The package now has no real PHPUnit coverage in the repository for its core behavior, and the Doctrine variant is especially exposed.

Current behavior

The component logic around discriminators, node transformation, prototype generation, and Doctrine-backed collections is largely unprotected by tests.

Expected behavior

The package should include PHPUnit coverage for:

• non-Doctrine polymorphic collections
• prototype generation
• discriminator failures
• DoctrinePolymorphicCollectionType behavior

Why it matters

This package is a critical building block for higher-level CMS features, so regressions here can affect many integrations.