refactor(web-impl): Improve DecisionFlowChartManagerImpl code quality

and maintainability

* Optimize code structure and reduce complexity in chart generation
* Extract common operations into reusable helper methods
* Add null checks and improved error handling
* Enhance stream operations efficiency
* Improve method organization and readability

DecisionFlowChartManagerImpl changes:
- Introduce helper methods for common data retrieval operations
- Simplify chart data addition logic
- Improve StringBuilder usage in summary generation
- Add null safety checks using Objects.requireNonNull
- Streamline committee lookup operations
- Optimize stream collectors usage
- Add logging capability for better debugging

Bug: None
Test: Existing tests should pass