admin: show 'need reorg' and 'validating' in admin show ddl jobs's comment field

## Enhancement
Currently, the `COMMENT` field in `ADMIN SHOW DDL JOBS` output does not provide enough information about whether a job is performing a data reorganization or just validating data (e.g., for lossy DDL optimization or varchar-to-char conversion).

This enhancement adds:
1. "need reorg" label to clarify if a job involves data reorganization.
2. "validating" label for jobs that skip reorganization but perform data validation (lossy DDL optimization, varchar-to-char).
3. Better summary logic for multi-schema change jobs.