[Feature] Add multi-query batch search support

[LHT129]

Description

Enable VSAG search operations to support multiple queries in a single call, leveraging the existing multi-element capability of DatasetPtr.

Background

Currently, search methods (KnnSearch, RangeSearch, SearchWithRequest) only accept a single query vector (NumElements=1). However, DatasetPtr already supports multiple elements through its NumElements API. Allowing batch queries would improve efficiency by reducing API call overhead and enabling better optimization opportunities.

Requirements

• Modify HGraph::SearchWithRequest to support multi-query search
• Update HGraph::KnnSearch and RangeSearch for batch operations
• Update SparseIndex::KnnSearch and RangeSearch for multi-query
• Update SINDI::KnnSearch and RangeSearch for multi-query
• Ensure backward compatibility (single query still works)
• Update error messages for clarity

Technical Details

• Remove `GetNumElements() == 1` restrictions
• Loop over each query vector using `get_data(query, q_idx)`
• Concatenate results: for N queries with k results each, return N*k elements
• IteratorContext-based search remains single-query only due to state tracking

Acceptance Criteria

• Multi-query search returns correct results for HGraph index
• Multi-query search returns correct results for SparseIndex
• Multi-query search returns correct results for SINDI
• Single query behavior unchanged
• Unit tests pass
• Release build successful

Related

• Original task file: batch_query.md
• Implementation plan: plan.md

Notes

• Results are concatenated in query order: [q0_0..q0_k-1, q1_0..q1_k-1, ...]
• IteratorContext-based search explicitly requires NumElements=1