Preserve definition-level defaults when types are reached via $ref

[danieleades]

Bug

When a schema definition carries its own default value, that default is dropped when the type is reached via $ref. For example:

{
  "definitions": {
    "DefaultedStruct": {
      "type": "object",
      "properties": { "a": { "type": "string" } },
      "default": { "a": "hello" }
    }
  }
}

Referenced via "$ref": "#/definitions/DefaultedStruct", the generated type loses the definition-level default (its Default impl falls back to per-field defaults, yielding a: None). Inlining the identical schema preserves it (a: Some("hello")).

Cause

convert_ref_type recomputes the default from the metadata returned by convert_schema and assigns it unconditionally. For struct conversions that returned metadata is None — TypeEntryStruct::from_metadata consumes the metadata and records the default itself — so the already-recorded default was clobbered with None. (Enums were unaffected because convert_enum_string returns Some(metadata), which is why the asymmetry went unnoticed.)

Fix

Only override the recorded default when the metadata returned by the conversion actually specifies one.

Precedence is preserved: a default declared at the reference site (sibling metadata alongside $ref, handled by the merge path) comes back as Some and still takes precedence over the definition-level default.

Testing

New golden test typify/tests/schemas/ref-with-default.json covering:

• a definition-level struct default reached via $ref — now generates Default for DefaultedStruct with a: Some("hello");
• a definition-level enum default reached via $ref — unchanged behavior;
• a ref-site override (allOf + sibling default) — still wins over the definition-level default.

No other golden outputs change. cargo test --workspace --locked passes; cargo fmt --all -- --check is clean.