Add baseline unit coverage for schema parsing and YAML generation

[Copilot]

The codebase had no tests around its highest-risk logic: OpenAPI schema resolution/parsing and YAML scaffold generation. This adds targeted unit coverage for those paths without changing runtime behavior.

• Schema parsing coverage

  • Adds unit tests in src/schema.rs for definition-key selection across matching strategies.
  • Covers v2 parsing with nested $ref resolution, required-field propagation, enums/defaults, arrays, and additionalProperties maps.
  • Covers v3 parsing through the existing Ctx flow to verify nested object expansion and map value typing.

• YAML generation coverage

  • Adds unit tests in src/generate.rs for filter_fields behavior in --minimal, default, and fallback cases.
  • Verifies the is_commonly_needed heuristic against representative included/excluded fields.
  • Exercises the YamlNode emitter for nested objects, arrays, inline comments, and free-form map placeholders.

• Approach

  • Keeps coverage close to the implementation with #[cfg(test)] modules in the two core files.
  • Uses small in-memory fixtures to lock down behavior without introducing new test infrastructure.

#[test]
fn filter_fields_default_keeps_commonly_needed_fields() {
    let fields = vec![test_field("image", false), test_field("debug", false)];

    let filtered = filter_fields(&fields, false, false);

    assert_eq!(filtered.len(), 1);
    assert_eq!(filtered[0].name, "image");
}