[Repo Assist] perf: use while! in groupBy, countBy, partition, except, exceptOfSeq

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Summary

This PR replaces the manual go-flag + initial-MoveNext pre-advance pattern with idiomatic while! in five functions: groupBy/groupByAsync, countBy/countByAsync, partition/partitionAsync, except, and exceptOfSeq.

The pattern being replaced (groupBy example)

Before:

let! step = e.MoveNextAsync()
let mutable go = step

while go do
    // body
    let! step = e.MoveNextAsync()
    go <- step

After:

while! e.MoveNextAsync() do
    // body

Changes

Function	Change
groupBy / groupByAsync	Remove go flag + initial pre-advance; use while! e.MoveNextAsync() per branch
countBy / countByAsync	Same pattern
partition / partitionAsync	Same pattern
except	Replace go flag with let! hasFirst = e.MoveNextAsync() + if hasFirst then + while! e.MoveNextAsync()
exceptOfSeq	Same as except; also uses while! excl.MoveNextAsync() to load exclusion set

Benefits

• Removes one mutable go bool per function
• Removes the initial pre-advance let! step = e.MoveNextAsync(); go <- step before each loop
• Unifies style across the module — groupBy, countBy, partition, except, and exceptOfSeq now match the established while! idiom already used in sum, sumBy, average, averageBy, iter, fold, reduce, etc.
• For except/exceptOfSeq, the first-element is also now handled before the main loop, making the code flow clearer

Scope

All changes are purely internal — no public API or behaviour change.

Trade-offs

None. This is a straightforward pattern replacement with identical semantics, verified by the full test suite.

Test Status

✅ Build: 0 warnings, 0 errors (Release)
✅ Fantomas: formatting check passes
✅ Tests: 5093 passed, 2 skipped, 0 failed

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