Fix invalid Rust emitted for defaults on types with flattened properties

[danieleades]

Problem

When generating a Rust value expression for a type-level default on a struct that has a #[serde(flatten)] property (e.g. an object with additionalProperties alongside declared properties), the code in typify-impl/src/value.rs interpolated the flattened property's name as a string literal instead of an identifier:

let name = &prop.name;
Some(quote! { #name: #flat_value })

Since name is a String, quote! renders it as a string literal ("extra": ...) rather than a field name, producing invalid Rust inside the struct literal. This causes rustfmt/compilation to fail for any generated default value on a struct with a flattened property.

Fix

Build a proper identifier for the flattened field, matching the sibling non-flattened path a few lines above (format_ident!("{}", &prop.name)).

Testing

Added typify/tests/schemas/flatten-default.json, a minimal schema with a struct-typed property carrying a default where the struct has both declared properties and additionalProperties (flattened into a map). This is picked up automatically by the existing test_schemas harness, which regenerates and rustfmts the output and then compiles it via trybuild.

• Confirmed on unpatched code this schema causes generation to fail with error: expected identifier, found "extra" (rustfmt).
• With the fix, cargo test --workspace --locked passes, including trybuild compilation of the new golden file.
• cargo fmt --all -- --check is clean.