server/proxy: simplify HTTP proxy domain registration by removing duplicate loop (#5207)

The Run() method had two nearly identical loop blocks for registering
custom domains and subdomain, with the same group/non-group registration
logic copy-pasted (~30 lines of duplication).

Consolidate by collecting all domains into a single slice first, then
iterating once with the shared registration logic. Also fixes a minor
inconsistency where the custom domain block used routeConfig.Domain in
CanonicalAddr but the subdomain block used tmpRouteConfig.Domain.