feat: Voyage AI rerank support on /v1/rerank with top_n/top_k + results/data adapter (#213 Phase 2.5)

Summary

PR #227 (#213 Phase 1) added Cohere on /v1/rerank. The Phase 2 PR adds Jina (also identity-mapped to the Cohere wire shape).

Voyage AI also exposes /v1/rerank at https://api.voyageai.com with Bearer auth, but its wire shape differs from Cohere/OpenAI/Jina in two material places:

1. Request — top_k instead of top_n: per https://docs.voyageai.com/reference/reranker-api, Voyage's reranker accepts top_k (integer) for the result-count cap. Cohere/Jina/OpenAI-compat use top_n. The gateway today forwards the body verbatim, so a caller sending {top_n: 5} would have Voyage return 400 unknown field "top_n".

2. Response — data instead of results: Voyage returns {object, data: [{index, relevance_score}], usage, model} whereas Cohere/Jina return {results: [{index, relevance_score, document?}], ...}. Caller code that iterates response.results would break on a Voyage upstream.

Both differences require a small request/response adapter that's out of scope for the rest of #213's surgical "relax the gate + add default api_base" pattern.

Suggested fix shape

In crates/aisix-proxy/src/rerank.rs::dispatch, after the existing model field rewrite and before the upstream POST:

// Per #213 Phase 2.5: Voyage's /v1/rerank diverges from the
// Cohere/Jina/OpenAI-compat wire shape on two fields. Translate
// caller-side names to Voyage's names on the request, then
// translate Voyage's response field names back on the way out.
if model.provider == Some(Provider::Voyage) {
    if let Some(map) = body.as_object_mut() {
        if let Some(top_n) = map.remove("top_n") {
            map.insert("top_k".to_string(), top_n);
        }
    }
}

And on the response side, after body_bytes:

if model.provider == Some(Provider::Voyage) {
    let mut json: serde_json::Value = serde_json::from_slice(&body_bytes)?;
    if let Some(obj) = json.as_object_mut() {
        if let Some(data) = obj.remove("data") {
            obj.insert("results".to_string(), data);
        }
    }
    body_bytes = serde_json::to_vec(&json)?.into();
}

(Sketch — actual implementation needs to handle the byte-stream → JSON parse → re-serialize round-trip carefully, and the response transform should preserve usage / model / other fields verbatim.)

Tests

Mirror the Cohere/Jina pattern:

• Rust unit test voyage_provider_dispatches_with_field_translation — pin caller's top_n becomes upstream top_k; pin upstream's data becomes caller-visible results.
• E2E case in rerank-e2e.test.ts — Voyage provider: gateway translates field names on both request and response (#213 Phase 2.5).

Updates required alongside

• Add Provider::Voyage enum variant + default_base_url/as_str arms.
• Add "voyage" to JSON schema enum.
• Add Voyage to default_base_for_provider in rerank.rs (https://api.voyageai.com).
• Update gate to accept {Openai, Cohere, Jina, Voyage}.
• Update docs §4.7 to list Voyage as supported, document the wire-shape adapter.

References

• Voyage Reranker API: https://docs.voyageai.com/reference/reranker-api
• Companion: #213 (Phase 1: Cohere — done; Phase 2: Jina — done; this issue is Phase 2.5: Voyage)

Start in crates/aisix-proxy/src/rerank.rs::dispatch and compare the existing Cohere/Jina dispatch tests. Add Voyage provider support, request and response field translation, the default URL, schema and gate updates, and documentation §4.7; run voyage_provider_dispatches_with_field_translation and the named Voyage case in rerank-e2e.test.ts to verify completion.