Repository config tool: mirrors without a trailing slash produce broken repo URLs (recurrence of MDBF-296)

The repository configuration tool on https://mariadb.org/download/ (Repository config tab) generates broken repository URLs for three mirrors. It looks like a recurrence of MDBF-296.

What happens

downloads.mariadb.org/rest-api/repo-instructions/ joins repo/... directly onto the mirror's mirror_url. These three mirrors are registered without a trailing slash, so the path is glued onto the host or directory:

The correct URL works, for example https://mariadb.spd.co.il/repo/11.8/ubuntu/dists/noble/Release returns 200. The other 81 mirror URLs in mirror-list-and-closest/?distro_type=deb&repo_series=11.8 end in a slash and are fine.

Reproduce: https://mariadb.org/download/?t=repo-config&d=24.04+%22noble%22&v=11.8&r_m=spd

Suggested fix

1. Fix the three mirror_url entries (add the trailing slash).
2. To keep it from coming back: join the path with a separator in the repo-instructions code (and anywhere else mirror_url is concatenated), or normalize mirror_url to end in / when a mirror is added. MDBF-296 was fixed in 2022, but new mirrors registered without a trailing slash break it again.

Reported by a user on GitHub: https://github.com/mariadb-corporation/mariadb-docs/issues/1386 (tracked internally as a docs ticket, which is closed as handed over to this one).