[Repo Assist] test: add missing side-effect and Seq-equivalence tests for sum/average#320

dsyme merged 3 commits intomainfsprojects/FSharp.Control.TaskSeq:mainfrom

repo-assist/test-sumby-sideeffects-2026-03-4e02cd9337359e5afsprojects/FSharp.Control.TaskSeq:repo-assist/test-sumby-sideeffects-2026-03-4e02cd9337359e5aCopy head branch name to clipboard