Fix the improper use of system variables in internal SQL

TiDB’s internal SQL reuse internal sessions (the sys session pool). If session-scoped system variables (sysvars) are modified on a reusable session but are not reliably restored on the same session, those changes may leak across tasks/requests into the next reuse, **causing subtle behavioral drift and hard-to-debug issues**. Use this issue as a single umbrella tracker to consolidate and track the related PRs.