Fix Voyage AI rerank response parsing

[vicksiyi]

Summary

• Parse the current Voyage AI rerank REST response shape from data.
• Preserve compatibility with the previous results response shape.
• Send the configured result limit as top_k and add regression coverage for both response shapes.

Fixes #6492

Tests

• pnpm --filter flowise-components exec jest nodes/retrievers/VoyageAIRetriever/VoyageAIRerank.test.ts --runInBand
• pnpm --filter flowise-components exec eslint nodes/retrievers/VoyageAIRetriever/VoyageAIRerank.ts nodes/retrievers/VoyageAIRetriever/VoyageAIRerank.test.ts --ext ts --max-warnings 0
• pnpm --filter flowise-components exec tsc --noEmit --pretty false

[gemini-code-assist]

Code Review

This pull request updates the VoyageAIRerank class to include top_k in the API request payload, support both the new data and legacy results response formats from the Voyage API, and use slice instead of splice to return the final results. Additionally, a new test suite has been added to verify these behaviors. The review feedback recommends adding defensive checks to safely parse the API response, including verifying that the results format is an array and ensuring that the documents exist at the returned indices before modifying their metadata.

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