Add hybrid work item children sorting implementation

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What does this MR do and why?

Add hybrid work item children sorting implementation

Implements a hybrid approach that combines the best aspects of MR 198784 and MR 193092 for work item children sorting:

• Uses legacy implementation for default sorting (relative_position_asc) to maintain optimal performance for common cases
• Uses WorkItemsFinder for non-default sorting to avoid sequential scans and leverage optimized query patterns
• Maintains backward compatibility with existing state filtering
• Feature flag controlled via work_item_children_filter_sort

Key improvements:

• No sequential scans for sorting operations (unlike MR 198784)
• Simplified implementation focused only on sorting (unlike MR 193092)
• Smart hybrid approach uses best method for each scenario
• Comprehensive test coverage with 19 test cases

Files changed:

• Add sort argument to ChildrenResolver with feature flag protection
• Create comprehensive test suite covering all scenarios
• Add feature flag configuration
• Update GraphQL API documentation

Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

References

Original issue: Add filtering and sorting to work item children (#526569)

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