sessionctx: fix TiDBGOGCTunerThreshold failing to ParseFloat (#38774)
sessionctx: fix TiDBGOGCTunerThreshold failing to ParseFloat (#38774)

close pingcap/tidb#38778
pingcap/tidb,pingcap/tidb,pingcap/tidb,pingcap/tidb,pingcap/tidb,pingcap/tidb