- Allow comma-separated stopword set names in search queries
- Implement combined stopword processing with `##COMBINED##:` prefix
- Merge all specified stopword sets into a single unified set
- Add validation to ensure all referenced stopword sets exist
- Add tests for multi-stopword set functionality

… handling

- Add `parse_stopword_set_names()` utility function in `StringUtils` for comma-separated parsing
- Remove `##COMBINED##:` prefix handling and simplify stopword set processing logic
- Refactor collection manager to store original parameter value instead of creating combined identifiers
- Consolidate stopword parsing logic in single location for better maintainability
- Use iterator-based insertion for combining multiple stopword sets

- instead of manually trimming whitespace, use the `split` function to
avoid redudant code

- add `get_combined_stopwords()` method to `StopwordsManager` class
- consolidate comma-separated stopword set parsing and merging logic
- replace inline stopword combination code in `collection.cpp`
- improve error handling for missing stopword sets
- reduce code duplication and improve maintainability

… parsing

- Replace combined stopwords processing with individual set iteration
- Parse stopword set names and process each set separately
- Add error logging for missing stopword sets
- Remove tokens from both `tokens` and `tokens_non_stemmed` arrays per set