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Description

Update Freezed code generation to use abstract classes for primary constructors, following the latest Freezed guidelines.
See: https://pub.dev/packages/freezed#primary-constructors

Related Issue

#2825

Motivation and Context

Recent versions of freezed require primary constructors to be defined in abstract classes.
This change ensures compatibility with newer Freezed versions.

Previous Behaviour / Output

Generated Freezed classes used concrete classes with primary constructors.

New Behaviour / Output

Generated Freezed classes now use abstract classes with primary constructors.

How Has This Been Tested?

Verified generated code compiles with the latest freezed.
Confirmed code generation works as expected.