codegen: implement open enum generation from enumMetadata

[bokelley]

Implement open-enum codegen using the PR #23 generator invariant.

Scope:

• Generate unknown-safe wrappers for open protocol vocabularies, defaulting to sealed Known<T> / Unknown(rawValue).
• Preserve raw unknown values exactly; no generic UNKNOWN sentinel that loses the wire string.
• Emit Jackson serializer/deserializer bindings for each open vocabulary so the wire shape remains a flat scalar string.
• Drive open/closed classification from schema metadata such as enumMetadata, not value count.
• Treat at least these vocabularies as open in the first pass:
  • ErrorCode
  • media_buy_status
  • creative_status
  • recovery
  • action-discovery enums in allowed_actions[] / available_actions[]
  • notification types, including wholesale-feed webhook events
  • task status values

References:

• PR docs: update roadmap for AdCP 3.1 parity #23
• ROADMAP.md Track 2
• specs/codegen-open-enums.md