Function set_nested_mapping 

pub fn set_nested_mapping(
    doc: Document,
    parent_path: &[&str],
    value_pairs: &[(&str, &str)],
) -> Result<Document>

Insert or replace a nested mapping at the given parent path.

parent_path is a sequence of mapping keys descending from the root. All but the last key must resolve to a mapping that the helper can either find in the source or create alongside its existing siblings. The last key (the leaf) is the mapping the caller wants to upsert. value_pairs becomes the body of that leaf mapping.

Existing siblings of the leaf — and existing siblings of any intermediate the helper has to create — are preserved with their comments and ordering intact. If the leaf mapping already exists, it is replaced wholesale by value_pairs. Other adapters under the same parent (e.g. discord: next to telegram:) survive.

Errors

Returns an error if parent_path is empty or if the first key is not a top-level mapping in the document.