Refine handling of POST /api/v1/datasets/search in GO (#15583)

### What problem does this PR solve?

Refine handling of POST /api/v1/datasets/search in GO

### Type of change

- [x] Refactoring

internal/cpp/darts_trie.cpp

@@ -43,6 +43,7 @@ int32_t POSTable::Load() {
         x.second = index++;
         pos_vec_.push_back(x.first);
     }
+    table_size_ = static_cast<int32_t>(pos_vec_.size());
     return 0;
     // return Status::OK();
 }