Function resolve_delete_behavior 

pub fn resolve_delete_behavior(nav: &NavigationMeta) -> DeleteBehavior

Resolves the effective DeleteBehavior for a navigation, applying EFCore-style defaults when delete_behavior is None:

• ManyToMany → Cascade (join rows are always pruned)
• required FK (non-nullable type, e.g. i32) → Cascade
• optional FK (nullable type, e.g. Option<i32>) → Restrict