Adds new codes to an existing coupon. This operation is additive —
existing codes are preserved and the new codes are appended.

Code handling

• Case: Codes are normalized to uppercase on storage. summer10
  and SUMMER10 are treated as the same code.
• Duplicates: Duplicate codes within a single request are
  accepted without error and echoed verbatim in the response, but
  storage silently dedupes to a single entry per unique
  (case-insensitive) code. Call
  GET /coupons/{coupon_id}/codes after a write to confirm the
  stored state.
• Format: Codes must contain only alphanumeric characters,
  hyphens, and underscores. Spaces and other special characters
  are rejected.