[Repo Assist] refactor: simplify lengthBy and lengthBeforeMax using while!

[github-actions]

🤖 This is an automated pull request from Repo Assist.

Summary

Simplify lengthBy and lengthBeforeMax by replacing the pre-loop MoveNextAsync + mutable go pattern with idiomatic while! loops.

Before

let mutable go = true
let mutable i = 0
let! step = e.MoveNextAsync()   // unconditional pre-loop advance
go <- step

match predicate with
| None ->
    while go do
        let! step = e.MoveNextAsync()
        i <- i + 1   // obscure ordering: count happens after advancing
        go <- step
| ...

After

let mutable i = 0

match predicate with
| None ->
    while! e.MoveNextAsync() do
        i <- i + 1
| ...

The while! e.MoveNextAsync() do idiom is the standard pattern used throughout the rest of this codebase (e.g. maxMin, sum, average, contains). Applying it here brings lengthBy in line with those functions and removes the confusing "count before moving" comment.

Improved semantics for lengthBeforeMax

The old implementation had an accidental "read-ahead by 1": it called MoveNextAsync once unconditionally before the loop, meaning TaskSeq.lengthOrMax 5 on a longer sequence would evaluate 6 elements instead of 5, and TaskSeq.lengthOrMax 0 would evaluate 1 element instead of 0.

The new implementation is cleaner and evaluates exactly min(length, max) elements — no read-ahead:

while go && i < max do
    let! hasMore = e.MoveNextAsync()
    if hasMore then i <- i + 1 else go <- false

Three tests in TaskSeq.Length.Tests.fs that explicitly asserted the old N+1 / 1 read-ahead counts have been updated to assert the correct N / 0 values, with revised test names and comments.

Test Status

Build: ✅ succeeded (0 warnings, 0 errors)
Tests: ✅ 5013 passed, 2 skipped (infrastructure), 0 failed

Branch: repo-assist/improve-length-clarity-20260317

Generated by Repo Assist · ◷

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@346204513ecfa08b81566450d7d599556807389f