fix: use msgspec.UNSET for optional typedef fields to omit them from serialization

Switches optional fields in TypeDef and subclasses from None defaults to
msgspec.UNSET with omit_defaults=True, ensuring absent fields are not
serialized as null. Adds serde contract tests to verify round-trip behaviour.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: cmgrote