Rationale for this change

A Union has no top-level nullability of its own; its nullability is derived from its variant dtypes. Generic DType::as_nullable and DType::as_nonnullable therefore need to propagate the requested nullability into every variant rather than returning the union unchanged.

This is split from #8739 so the DType behavior can be reviewed independently from UnionScalar support.

What changes are included in this PR?

• make DType::with_nullability propagate through UnionVariants
• add UnionVariants::with_nullability
• cover nullable and non-nullable round trips at the dtype layer

What APIs are changed? Are there any user-facing changes?

UnionVariants::with_nullability is a new public API. Calling DType::as_nullable, DType::as_nonnullable, or DType::with_nullability on a union now updates every variant dtype.

Validation

• cargo nextest run -p vortex-array test_with_nullability_propagates_to_variants
• cargo +nightly fmt --all -- --check
• cargo clippy -p vortex-array --all-targets --all-features

Workspace-wide Clippy could not complete locally because the DuckDB build could not find duckdb.h.