[Repo Assist] Fix traverseOptionAsync/traverseChoiceAsync — avoid wasteful MoveNext on failure

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Summary

Refactors AsyncSeq.traverseOptionAsync and AsyncSeq.traverseChoiceAsync to fix a subtle over-enumeration bug and improve readability.

The bug

Both functions used a while loop structured like this:

while b.Value.IsSome && not fail.Value do
    let! vOpt = f b.Value.Value
    match vOpt with
    | Some v -> buffer.Add v
    | None -> b := None; fail := true   // set "stop" flags ...
    let! moven = ie.MoveNext()          // ... but STILL read next element!
    b := moven

When f returns None (or Choice2Of2), the code correctly set the exit flags — but then unconditionally called ie.MoveNext() anyway, reading one extra element from the source that would never be used. For sequences with observable side effects on enumeration (logging, side-effecting generators) this was surprising and incorrect.

The fix

The MoveNext() call is moved inside the success branch so it only runs when we actually need the next element:

while current.IsSome && not failed do
    let! vOpt = f current.Value
    match vOpt with
    | Some v ->
        buffer.Add v
        let! next = ie.MoveNext()   // only advance when still consuming
        current <- next
    | None ->
        failed <- true              // exit without reading further

ref cells are also replaced with mutable locals, and ValueOption is used for the error holder in traverseChoiceAsync to avoid heap-allocating a Some wrapper on every run.

New tests

Test	Description
traverseOptionAsync returns Some sequence when all elements succeed	Happy path — mapped values are correct
traverseOptionAsync does not read past failing element	Source enumerator is advanced exactly N times when failure occurs at element N
traverseChoiceAsync returns Choice1Of2 sequence when all elements succeed	Happy path
traverseChoiceAsync does not read past failing element	Same no-over-read guarantee

Test Status

✅ Build succeeded (0 errors, pre-existing warnings only — NU1605, FS9999 for groupByAsync).

✅ All 325 tests pass — 4 new, 321 pre-existing.

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