Retrieve Chunks Grouped

Retrieve relevant chunks with grouped response format.

Uses the same filter operators as /retrieve/chunks (equality, $eq/$ne, $gt/$gte/$lt/$lte, $in/$nin, $exists, $type, $regex, $contains, and the logical $and/$or/$nor/$not), with arbitrary nesting supported inside request.filters.

Returns both flat results (for backward compatibility) and grouped results (for UI). When padding > 0, groups chunks by main matches and their padding chunks.