fix(admin): keep the gateway key inventory off the s3s surface (#7817)

The gateway key inventory handler added in #7785 imported s3s directly and built four errors with s3_error!, pushing the s3s footprint ratchet to 210 files and 1592 s3_error! lines on main. Route its S3 types and error construction through the crate::admin::storage_api::s3 facade like sibling admin handlers; the codes, HTTP statuses and messages are unchanged, and a new unit test pins them.

Co-authored-by: cxymds <cxymds@gmail.com>
Co-authored-by: Hauser <housemecn@gmail.com>