feat: add ZeroEntropy reranker

[flamerged]

Target: danny-avila/agents:main
Branch: feature/zeroentropy-reranker
Commit: b673d8103cea9b8d0d86ca63964658df3de935e0

Companion PR:

• Companion LibreChat PR: danny-avila/LibreChat#13263
• Companion branch: feat/zeroentropy-reranker
• Companion target: dev
• Merge/publish order: merge and publish this agents package before merging the LibreChat ZeroEntropy plumbing PR.

Why This Matters

ZeroEntropy documents zerank-2 as its flagship state-of-the-art reranker. Adding it here gives LibreChat's agent search path access to a modern cross-encoder reranker that can improve result ordering after first-pass retrieval, with configurable API URL/model support for teams that want ZeroEntropy Cloud, EU endpoints, or self-hosted/commercial-license deployments.

This is a high-leverage upgrade for web search and RAG-style tool use: better reranking means the model sees more relevant sources earlier, without changing the search provider itself.

References:

• ZeroEntropy model docs: https://docs.zeroentropy.dev/models
• ZeroEntropy rerank API: https://docs.zeroentropy.dev/api-reference/models/rerank

Summary

Adds a ZeroEntropy reranker implementation using the rerank API, including:

• zerank-2-ready request/response mapping
• ZeroEntropy reranker config fields
• createSearchTool / createReranker wiring
• focused coverage for request shape, response mapping, and config passthrough

Tests

• NODE_OPTIONS='--experimental-vm-modules' npx jest src/tools/search/jina-reranker.test.ts src/tools/search/tavily.test.ts --runInBand: pass, 47 tests
• npx tsc --noEmit: pass
• npx eslint src/tools/search/rerankers.ts src/tools/search/types.ts src/tools/search/tool.ts src/tools/search/jina-reranker.test.ts src/tools/search/tavily.test.ts: pass
• npm run build: pass, existing Rollup empty-chunk warning only
• git diff --check: pass

Dependency Notes

No separate ZeroEntropy SDK dependency is added; implementation uses existing HTTP client utilities. The companion LibreChat PR requires a published @librechat/agents version that supports rerankerType: "zeroentropy", zeroEntropyApiKey, zeroEntropyApiUrl, and zeroEntropyModel.

Residual Risks

No live ZeroEntropy API validation was run.