[YSQL] QPM clock LRU can return an empty slot after a reset, and the next insert crashes

reset all;

SET yb_pg_stat_plans_max_cache_size = 3;  -- postmaster GUC; set in ysql_pg.conf / tserver flag
SET yb_pg_stat_plans_cache_replacement_algorithm = simple_clock_lru;

SELECT yb_pg_stat_plans_reset(NULL, NULL, NULL, NULL);

-- Fill slots 0,1,2 (A,B,C)
SELECT yb_pg_stat_plans_insert(1,1,1,1, 'h','p', now(), now(), 1, 1);
SELECT yb_pg_stat_plans_insert(1,1,1,2, 'h','p', now(), now(), 1, 1);
SELECT yb_pg_stat_plans_insert(1,1,1,3, 'h','p', now(), now(), 1, 1);

-- Eviction sweep: clears referenced on B,C; replaces A with D; clockHand = 1
SELECT yb_pg_stat_plans_insert(1,1,1,4, 'h','p', now(), now(), 1, 1);

-- Hole at C's slot (2), not at clockHand
SELECT yb_pg_stat_plans_reset(1, 1, 1, 3);

-- Steals unreferenced B at slot 1; hole at 2 remains
SELECT yb_pg_stat_plans_insert(1,1,1,5, 'h','p', now(), now(), 1, 1);

-- HASH_ENTER makes max+1; clockHand on the hole → old code SIGSEGV
SELECT yb_pg_stat_plans_insert(1,1,1,6, 'h','p', now(), now(), 1, 1);