Return the immediate child tree nodes for a given parent node.

Designed for per-click lazy loading of the dataset dimension tree. Each navigation step sends the parent_dimension_key of the node just drilled into and receives back its direct children — a single btree scan regardless of total dataset size.

Args: dataset_id: The BackfillVersionMetadata ID (UUID). parent_dimension_key: The dimension_key of the parent BackfillVersionFile row (from a previous TreeNodeResponse), encoded as "k=v&k=v" with percent-encoded values. Pass "" (or omit) for the root level — returns all depth-1 nodes for the dataset. asset_type: Optional filter to restrict results to a specific asset type (e.g. "Currency"). Used to split shared datasets such as headline_asset_sentiment_index across product tabs. sector: Optional filter to restrict results to a specific sector (e.g. "Energy").