Make DefaultFilterForm robust for untyped and non-Doctrine properties

[javihgil]

Problem

DefaultFilterForm currently builds filter fields and sortable fields from raw reflection properties.

Current behavior

• order_valid_fields is built from all reflected properties of the entity class.
• getFilterFields() iterates all reflected properties as potential filters.
• String detection calls ReflectionProperty::getType()->getName() without guarding null.

Why it matters

This can break or behave incorrectly when an entity has:

• untyped properties
• transient or internal properties that are not Doctrine fields
• properties that should not be exposed as sortable or filterable fields

Suggested direction

Base the default filter and default sortable fields on Doctrine metadata or add stronger guards so the generated defaults only include safe, supported fields.