fix: case when reranker returns invalid scores array

[uinstinct]

Description

The following error used to happen when the reranker model returned an invalid response.
Failed to rerank retrieval results TypeError: Cannot read properties of undefined (reading 'sort'). More about the RCA in the ticket.

• add a zod validation schema for voyage ai response
• check if scores array is valid in DocsContextProvider

resolves CON-2661

Checklist

• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Tests

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Summary by cubic

Fixed a crash when the reranker model returned an invalid scores array by adding response validation and handling invalid cases.

• Bug Fixes
  • Added Zod schema validation for the Voyage AI reranker response.
  • Checked for a valid scores array before sorting in DocsContextProvider.