Scalar hashing violates Eq for nested nullability #8744

What happened?

Scalar::eq compares dtypes with DType::eq_ignore_nullability, which ignores nullability recursively. Scalar::hash hashes dtype.as_nonnullable(), which only removes top-level nullability.

As a result, equal scalars with nested dtypes can produce different hashes, violating Rust's Eq/Hash contract. This affects nested List, FixedSizeList, Struct, Union, and extension storage dtypes.

The typed views appear to have the same issue: ListScalar, StructScalar, and ExtScalar compare dtypes ignoring nullability but hash their raw dtypes.

Steps to reproduce

use std::collections::hash_map::DefaultHasher;
use std::hash::{Hash, Hasher};

use vortex_array::dtype::{DType, Nullability, PType};
use vortex_array::scalar::Scalar;

fn hash(value: &Scalar) -> u64 {
    let mut hasher = DefaultHasher::new();
    value.hash(&mut hasher);
    hasher.finish()
}

let nullable = Scalar::list(
    DType::Primitive(PType::I32, Nullability::Nullable),
    vec![Scalar::primitive(42_i32, Nullability::Nullable)],
    Nullability::NonNullable,
);
let non_nullable = Scalar::list(
    DType::Primitive(PType::I32, Nullability::NonNullable),
    vec![Scalar::primitive(42_i32, Nullability::NonNullable)],
    Nullability::NonNullable,
);

assert_eq!(nullable, non_nullable);
assert_eq!(hash(&nullable), hash(&non_nullable)); // fails

Environment

• Vortex version: current develop
• Rust
• All platforms

Additional context

DType::with_nullability only replaces the outer nullability, while DType::eq_ignore_nullability recursively compares nested dtypes.

Hashing needs to use the same nullability-insensitive equivalence relation as equality, or the equality contract needs to be narrowed.