Avoid clippy::unit_arg in generated builder code for unit-typed properties

[danieleades]

Problem

For struct properties whose type is the unit type () (produced by null schemas), builder-mode codegen emits expressions that pass a unit value to Ok(..):

• in impl Default for builder::T: field: Ok(Default::default())
• in impl From<super::T> for builder::T: field: Ok(value.field)

Both trip clippy's warn-by-default unit_arg lint. Generating builder-mode output for the typify-impl/tests/github.json fixture produces 105 such warnings. cargo-typify's generated #![allow(..)] prelude doesn't cover unit_arg, and consumers using the macro or build.rs get the warnings in their own builds.

Fix

Detect unit-typed properties in output_struct and emit Ok(()) directly in the builder's Default and From impls. This is semantically identical: reading or defaulting a unit field always yields () and has no effects. For an all-unit struct, the generated conversion names its otherwise-unused source parameter _value.

Testing

• Added the committed builder-unit-property schema/output pair, which exercises builder mode and shows Ok(()) in both generated implementations.
• Compiled the github.json builder output with its generated lint-allow prelude removed under cargo clippy --all-targets -- -D clippy::unit_arg; no unit_arg diagnostics remain.
• cargo test --workspace --locked passes and cargo fmt --all -- --check is clean.

Trade-offs

None behavioral- only the spelling of generated initializers and an unused all-unit conversion parameter changes.

[danieleades]

not sure the juice is worth the squeeze on this one