value for "zip" omits leading zeros

When querying the endpoint by zip code and year (https://api.gsa.gov/travel/perdiem/v2/rates/zip/{zip}/year/{year}) the response data omits the leading 0 in the "zip" field. For example, calling the endpoint https://api.gsa.gov/travel/perdiem/v2/rates/zip/07001/year/2025, the result returns the "zip" value as "7001".

Start by requesting the documented endpoint /travel/perdiem/v2/rates/zip/{zip}/year/{year} with zip 07001 and year 2025, then trace the response handling for the "zip" field. Done means the response preserves the leading zero and the same behavior is verified for ZIP codes with leading zeros.

Written by the indexing model from the issue text.