ddl: Fix default value filling with finer granularity (#10682)#10689

JaySon-Huang merged 5 commits intopingcap:release-7.5-20260128-v7.5.6pingcap/tiflash:release-7.5-20260128-v7.5.6from

ti-chi-bot:cherry-pick-10682-to-release-7.5-20260128-v7.5.6ti-chi-bot/tiflash:cherry-pick-10682-to-release-7.5-20260128-v7.5.6Copy head branch name to clipboard