Fix: Refactor parser config handling and add GraphRAG defaults (#8778)
### What problem does this PR solve?
- Update `get_parser_config` to merge provided configs with defaults
- Add GraphRAG configuration defaults for all chunk methods
- Make raptor and graphrag fields non-nullable in ParserConfig schema
- Update related test cases to reflect config changes
- Ensure backward compatibility while adding new GraphRAG support
- #8396
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)