Fix Last() on primitive collections returning the first array element

[sanjaysingh]

Summary

• Last() and LastOrDefault() on a PostgreSQL array were translated like First(). The unnest cleanup dropped a leading ordinality ordering because unnest already returns rows in ascending order, and that also removed the descending sort Last() needs.
• Keep WITH ORDINALITY and ORDER BY ordinality DESC when that ordering is descending. Ascending ordinality is still removed, so First() SQL is unchanged.
• Fixes Last()/LastOrDefault() on primitive collections is translated like First() — missing descending ordinality ordering #3909

Test plan

• dotnet build for EFCore.PG and EFCore.PG.FunctionalTests

[roji]

Thanks, LGTM!