Function plan_nested_batch_fetch 

pub fn plan_nested_batch_fetch(
    relations: &RelationRegistry,
    parent_table: &str,
    related_table: &str,
    parent_keys: Vec<Value>,
) -> Result<Option<NestedBatchPlan>, BatchPlanError>

Plan a batched nested-relation fetch using relation metadata and parent keys.

Returns Ok(None) when the provided keys are empty after null-elision.