Add validation for coupon description in BmsCoupon model

- Introduced a new field validator for the "description" field in the BmsCoupon class.
- Ensures that the description is limited to a maximum of 12 characters, enhancing input validation and error handling.

Author: Palbahngmiyine

solapi/model/kakao/bms/bms_coupon.py

@@ -53,3 +53,12 @@ def validate_coupon_title(cls, v: Optional[str]) -> Optional[str]:
                 '"OOO UP 쿠폰" (7자 이내)'
             )
         return v
+
+    @field_validator("description")
+    @classmethod
+    def validate_coupon_description(cls, v: Optional[str]) -> Optional[str]:
+        if v is None:
+            return v
+        if len(v) > 12:
+            raise ValueError("쿠폰 설명은 최대 12자 이하로 입력해주세요.")
+        return v