Collect trace logs in major hold RCA

[wayyoungboy]

Summary

• extract trace IDs from major hold diagnose information and pending DDL tasks
• collect matching observer.log* and rootservice.log* entries for each trace ID across observer nodes
• report when no matching logs are found, and add unit coverage for trace log collection

Fixes #1239

Verification

• PYTHONPATH=. python -m pytest test/rca/test_major_hold_trace_logs.py test/common/test_scene.py -q
• black --check -S -l 256 plugins/rca/major_hold.py test/rca/test_major_hold_trace_logs.py
• python workflow_data/check_py_files.py .
• git diff --check