We require reqStats to have only 1 element in createPartialStatsPlan()

This works for CREATE STATISTICS <stat_name> FROM <table_name> USING EXTREMES where we don't specify a column since we plan and run create stats separately for each column in the default set. However, this doesn't support EXPLAIN ANALYZE CREATE STATISTICS <stat_name> FROM <table_name> USING EXTREMES and certain other EXPLAINS (such as EXPLAIN (VEC)) since we pass job details with all of the columns in the default column set together and expect a single plan:

Jira issue: CRDB-41268