Function gather_qualified_columns 

fn gather_qualified_columns<'a>(
    prefix_text: &str,
    file_object: &FileObject,
    project_cache: Option<&ProjectCache>,
    types_lock: &Types,
    default_db: &str,
    default_schema: &str,
) -> Vec<CompletionCandidate<'a>>

Gather columns from a specific qualified object reference.

Splits the prefix at the last . into (object_text, col_filter), resolves object_text to an ObjectId via resolve_qualified_object, checks it is a dependency, and returns its columns filtered by col_filter.