feat: improve type inference for related fields

[legau]

Description

This PR improves typing for relational field factories so passing a model class preserves the concrete related model type in the returned relation.

Motivation and Context

Currently, relational field factories expose a free type variable but accepted type[Model] | str, which prevents type checkers from inferring the concrete related model when a real model class was passed.

This meant code like fields.ForeignKeyField(Field, related_name="segments") would not resolve to ForeignKeyRelation[Field].

How Has This Been Tested?

Added a type checking test file

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.