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.