Enhance execution status system to track skipped steps

[chrisli30]

Problem

The current execution status system has limitations in tracking skipped steps. Individual steps only have boolean success field, not status enum. Steps skipped due to unmet branch conditions, filter conditions, or unconnected nodes are not recorded in ExecutionLogs.

Impact

• Workflows with skipped steps show SUCCESS instead of more accurate status
• Missing audit trail for workflow execution paths
• Difficult debugging of conditional execution

Proposed Solution

1. Add step-level status enum (SUCCESS/FAILED/SKIPPED/PENDING)
2. Record skipped steps in ExecutionLogs with reasons
3. Update AnalyzeExecutionResult to consider skipped steps
4. Maintain backward compatibility

This builds on the recent ExecutionStatus enum migration for overall execution status.