fix: honour required contract on serialize for fields with intrinsic Rust defaults

[ndreno]

Summary

When a schema marks a field `required` but its Rust type has an intrinsic default (`Vec`, `HashMap`, `Option`), the previous behaviour (from #918) emitted both `#[serde(default)]` and `skip_serializing_if`. Deserialize was lenient (`{}` parsed), but serialize silently dropped the field when it was empty — violating the schema's wire contract that a required field must always be present.

This PR introduces `StructPropertyState::RequiredWithDefault`, which splits the two halves: emit `#[serde(default)]` alone, keeping deserialize lenient while serialize always renders the field.

Before / after

For schema `{ required: ["tags"], properties: { tags: { type: "array" } } }` and value `Foo { tags: vec![] }`:

	Before	After
Deserialize `{}`	OK	OK
Serialize `Foo { tags: vec![] }`	`{}`	`{"tags":[]}`
Round-trip	lossy (loses `tags` key)	lossless

Wire-format change

This is a breaking change for downstream JSON consumers. Any code that ingests serialized output from typify-generated types — diff tooling, snapshot tests, mock services, contract tests — will see required fields appear where they were previously omitted (`[]`, `{}`, or `null`). Payload size grows modestly for schemas with many required nullable/array fields (the GitHub OpenAPI fixture moves 908 lines for this reason).

Documented under Bug fixes in `CHANGELOG.adoc`.

Tests

• Codegen fixture `typify/tests/schemas/required-with-implicit-defaults.json` — three schemas covering required Vec / Map (with and without explicit `default: []`/`{}`) and a mixed required-and-optional struct. `expectorate` verifies the generated output.
• Five runtime tests in `typify-test/src/main.rs`:
  • `test_required_implicit_defaults_deserialize_empty` — `from_str("{}")` succeeds, fields use intrinsic empties.
  • `test_required_implicit_defaults_deserialize_partial` — partial JSON fills the gap.
  • `test_required_implicit_defaults_serialize_emits_required_fields` — regression-critical: required empty Vec/Map still appear in serialized output.
  • `test_mixed_required_and_optional_serialization` — required emits, optional empty still skipped.
  • `test_required_implicit_defaults_roundtrip` — content survives serialize → deserialize.
• Existing fixtures regenerated (intentional): `github.out` (-908), `vega.out` (-22), `draft-2019-09.rs`, `draft-2020-12.rs`, `property-pattern.rs`, `rust-collisions.rs`, `untyped-enum-with-null.rs`. Each diff replaces `#[serde(default, skip_serializing_if = "...")]` with `#[serde(default)]` exactly where the field is in the schema's required array.

Test plan

• `cargo fmt --all -- --check`
• `cargo clippy --workspace --all-targets --locked --exclude typify-test` (only pre-existing dead-code warnings, addressed in PR ci: cover fmt, clippy, tests, doc, advisories, and MSRV #3)
• `cargo test --workspace --locked` — all suites pass, including the 5 new runtime tests
• `cargo deny check advisories` (no new advisories)