Fix Float16 vector query execution

[Copilot]

The QueryExecutor::Execute() method was missing Float16 vector type handling, causing queries with float16 vectors to fail with "Unknown vector type: 2" error.

Changes

• Added Float16 case to constraint.cpp:QueryExecutor::Execute() following the BFloat16 pattern:
  • Quantize query vector using QuantizeToF16()
  • Handle both normalized and non-normalized query paths
  • Pass quantized vector to HNSW index search

} else if (space_.vector_type == VectorType::Float16) {
  F16Vector quantized_vector = QuantizeToF16(knn_param->query_vector);
  
  if (!space_.normalize) {
    return index_.searchKnnCloserFirst(quantized_vector.data().data(),
                                       knn_param->k, rowid_filter.get());
  }
  
  VECTORLITE_ASSERT(space_.normalize);
  F16Vector normalized_vector = quantized_vector.Normalize();
  return index_.searchKnnCloserFirst(normalized_vector.data().data(), 
                                     knn_param->k, rowid_filter.get());
}

This completes Float16 support across the query execution path.

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