Follow-up on the salvaged #56392 guard. The cherry-picked change matched
custom:<name> pool entries against the primary by raw base_url string
equality, which (a) can't disambiguate two named custom providers sharing
one gateway base_url and (b) left a latent bare-"custom" entry bypass.
Route the match through get_custom_provider_pool_key(rt[base_url]) compared
against the entry's custom:<name> key, mirroring the sibling guard in
recover_with_credential_pool. Use CUSTOM_POOL_PREFIX instead of the literal.
Add regression tests for the custom same-endpoint (swap) and cross-endpoint
(skip) branches, plus the plain-provider fallback-pool case from #56885.
kshitijk4poor
b837f07dcd