MB-69881: Re-architect vector search

[CascadingRadium]

• Adapt to new SegmentEligibleDocuments interface which allows usage of visitor-pattern based iteration of eligible documents for filtered vector search.

[Copilot]

Pull request overview

This PR refactors the vector search architecture to adopt a visitor-pattern based iteration model for eligible documents in filtered vector searches. The change adapts the existing code to implement the new SegmentEligibleDocuments interface from the bleve_index_api package.

Key Changes:

• Replaced direct slice return (SegmentEligibleDocs) with an iterator-based approach (SegmentEligibleDocuments)
• Introduced eligibleDocumentList and eligibleDocumentIterator types to support the new visitor pattern
• Updated VectorReader to use the new Count() method instead of directly checking slice length

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