### What problem does this PR solve?
Fixes#15012
The Novita Go driver landed in #14850 and shipped a stub `Rerank` method
that returned `"novita, no such method"`, so Novita could not be used as
a rerank provider in RAGFlow. This PR fills that gap, in the same way
#14895 filled the Embed gap on the same driver.
Novita exposes a public rerank endpoint at `POST
https://api.novita.ai/openai/v1/rerank` that accepts the
Cohere-compatible request shape (`{model, query, documents, top_n}`)
with `Authorization: Bearer <api_key>`. `baai/bge-reranker-v2-m3` is
documented in Novita's model library with a 1024-token limit.
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