[Repo Assist] refactor: simplify splitAt 'rest' taskSeq to use while!

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🤖 This is an automated pull request from Repo Assist, an AI assistant.

Summary

The rest sequence produced by TaskSeq.splitAt used a manual go2 flag and an explicit MoveNextAsync pre-advance before its inner loop:

letrest= taskSeq {let mutablego2= go
if go2 thenlet!step= e.MoveNextAsync()
go2 <- step
while go2 doyield e.Current
let!step= e.MoveNextAsync()
go2 <- step
}

This is equivalent to a single while! e.MoveNextAsync() do guarded by the existing outer go flag (which already tracks whether the source was exhausted while filling first):

letrest= taskSeq {if go thenwhile! e.MoveNextAsync()doyield e.Current
}

This follows the same while! idiom used throughout the codebase (e.g. tryTail, tryLast, lengthBy).

Correctness

Three cases:

Scenario	go	Current behaviour	New behaviour
Source exhausted before count	false	rest is empty	rest is empty (guard skips)
Source has ≥ count elements	true	Advance past first, yield remainder	while! does the same advance-then-yield
count = 0	true	Advance from start, yield all	Same

Test Status

• Build: ✅ dotnet build -c Release — succeeded (0 warnings, 0 errors)
• Tests: ✅ dotnet test -c Release — 5180 passed, 2 skipped (pre-existing infrastructure skips)
• Format: ✅ dotnet fantomas . --check — passed

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