feat(macros): nullable column attribute for non-Option<T> Rust types

[vindard]

Summary

Column::is_optional() checks the Rust type syntactically — true only when the type literally begins with Option<. That misses a real shape: a domain type whose custom sqlx::Encode impl maps one variant to SQL NULL (e.g. an enum Finite(Decimal) | Infinite that encodes Infinite as IsNull::Yes). Such columns are nullable in SQL but look non-optional to the macro, so they receive the non-nullable cursor SQL form which silently drops NULL rows from page 2+.

The downstream workaround (see GaloyMoney/lana-bank#5898) is to declare ty = "Option<Decimal>" at the storage edge and convert to/from the domain type via a boundary accessor. That works but leaks storage shape into the cursor and the repo accessor, and introduces a separate type at every call site.

Stacks on #137 (direction-aware NULL fallback in cursor condition).

Fix

Add an opt-in nullable attribute on the column declaration:

columns(
    loan_to_collateral_ratio(
        ty = "LoanToCollateralRatio",
        nullable,
        list_by,
        // ...
    ),
)

Now the macro detects nullability from either signal: the syntactic Option<T> shape OR the explicit annotation. Two methods carry the two semantics:

• Column::is_optional() — Rust type is Option<T>. Drives query parameter casting (avoid double-wrapping in Option) and cursor destructuring (the field is already optional).
• Column::is_nullable_column() — SQL column can hold NULL. Returns true if either is_optional() or nullable. Drives ORDER BY ... NULLS FIRST/LAST emission and the nullable-aware cursor WHERE form (see fix(macros): direction-aware NULL fallback in cursor condition for nullable sort columns #137).

For a column declared ty = "DomainEnum", nullable:

• SQL shape: nullable-aware (NULLS FIRST/LAST, IS NOT DISTINCT FROM, direction-aware NULL fallback from fix(macros): direction-aware NULL fallback in cursor condition for nullable sort columns #137 — same as Option<T>).
• Query parameter binding: value as Option<DomainEnum> (else branch of query_arg_tokens). sqlx encodes the variant that maps to IsNull::Yes as NULL, the rest as their normal payload.
• Cursor field type: the raw DomainEnum (no Option wrap in the cursor struct — keeps the domain type at the read boundary).

Downstream impact

Once both #137 and this PR land + a release ships, GaloyMoney/lana-bank#5898 can revert its ty = "Option<rust_decimal::Decimal>" workaround back to the cleaner ty = "LoanToCollateralRatio", nullable and restore the custom sqlx::Encode impls on the domain type.

Test plan

• New unit test list_by_fn_nullable_attribute_emits_nullable_aware_sql_for_non_option_type verifies the emitted SQL matches Option<T>'s (NULLS FIRST/LAST, IS NOT DISTINCT FROM, direction-aware fallback) while the parameter binding stays as Option<DomainEnum>.
• All 83 macro tests pass (82 existing + 1 new).

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