Go: implement Rerank in Replicate driver (#15278)

### What problem does this PR solve?

`ReplicateModel.Rerank` in `internal/entity/models/replicate.go` was a
`"replicate, no such method"` stub. The chat path landed in #14958 and
the embed path in #15073; rerank is the last major retrieval surface
still missing on this provider.

Until this PR, a tenant who selected a Replicate reranker model got the
sentinel error on every rerank call.

Co-authored-by: sxxtony <sxxtony@users.noreply.github.com>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>