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

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The Iterator method returns a value type (eligibleDocumentIterator) that shares the underlying array with docNums. If the caller mutates the iterator's slice, it could inadvertently modify the original eligibleDocumentList's data. Consider returning a pointer to avoid this potential issue and to support proper state management in the iterator's Next method.

Suggested change

	return eligibleDocumentIterator(edl.docNums)
	}
	type eligibleDocumentIterator []uint64
	func (edi eligibleDocumentIterator) Next() (uint64, bool) {
	if len(edi) == 0 {
	return 0, false
	}
	rv := edi[0]
	edi = edi[1:]
	return &eligibleDocumentIterator{
	docNums: edl.docNums,
	pos: 0,
	}
	}
	type eligibleDocumentIterator struct {
	docNums []uint64
	pos int
	}
	func (edi *eligibleDocumentIterator) Next() (uint64, bool) {
	if edi == nil || edi.pos >= len(edi.docNums) {
	return 0, false
	}
	rv := edi.docNums[edi.pos]
	edi.pos++