Better handling of trailing `/` in federation prefixes

[jhiemstrawisc]

A user tried registering a namespace like /foo/bar/, which generated an error. The correct registration was /foo/bar without a trailing /.

I do think it's important that our internal representations of prefixes contain a trailing / so that it's much harder to accidentally match /foo/barbaz to /foo/bar, but we can make this logic transparent to users by permitting registration of both /foo/bar and /foo/bar/ (and treating them the same under the hood).

[jhiemstrawisc]

I saw this again today -- a multi-export origin had /ndp/burnpro3d-auth/ as a namespace. It appears that it registered/advertised okay the first time it booted with the namespace, but then after modifying the namespace capabilities (triggering a reboot), the Origin's automatic registration step failed:

time="2025-04-29T19:32:15Z" level=error msg="Server responded with an error: Server encountered an error during key-sign challenge: Failed to add the prefix \"/ndp/burnpro3d-auth\" to the database: constraint failed: UNIQUE constraint failed: namespace.prefix (2067). . The POST attempt to https://osdf-registry.osg-htc.org/api/v1.0/registry resulted in status code 500"
time="2025-04-29T19:32:15Z" level=error msg="Failed to register with namespace service: Failed to register prefix /ndp/burnpro3d-auth/: Server responded with an error: Server encountered an error during key-sign challenge: Failed to add the prefix \"/ndp/burnpro3d-auth\" to the database: constraint failed: UNIQUE constraint failed: namespace.prefix (2067). : The POST attempt to https://osdf-registry.osg-htc.org/api/v1.0/registry resulted in status code 500; will automatically retry in 10 seconds\n"