[Repo Assist] Fix: removeAt and updateAt raise ArgumentException for out-of-range index

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Summary

AsyncSeq.removeAt and AsyncSeq.updateAt silently returned the source sequence unchanged when the given index was ≥ the sequence length. This inconsistency with the rest of the F# standard library could lead to subtle bugs.

Root Cause

The implementations iterated the sequence and simply skipped the operation when the index was never reached, with no post-loop validation:

// before — silently does nothing if index >= length
let removeAt (index : int) (source : AsyncSeq<'T>) : AsyncSeq<'T> = asyncSeq {
    if index < 0 then invalidArg "index" "must be non-negative"
    let mutable i = 0
    for x in source do
      if i <> index then yield x
      i <- i + 1 }   // ← no check after the loop

Fix

After the enumeration loop, check whether i <= index (meaning the element at index was never visited) and raise ArgumentException with the same message already used by AsyncSeq.insertAt:

    if i <= index then
      invalidArg "index" "The index is outside the range of elements in the collection."

Consistency

Function	Negative index	Out-of-range index
List.removeAt	✅ raises	✅ raises
Array.removeAt	✅ raises	✅ raises
AsyncSeq.insertAt	✅ raises	✅ raises
AsyncSeq.removeAt (before)	✅ raises	❌ silently ignores
AsyncSeq.updateAt (before)	✅ raises	❌ silently ignores
AsyncSeq.removeAt (after)	✅ raises	✅ raises
AsyncSeq.updateAt (after)	✅ raises	✅ raises

Trade-offs

This is technically a breaking change for any code that relied on the (buggy) silent-ignore behaviour. However, that behaviour was clearly unintentional and inconsistent with every comparable function.

Test Status

✅ Build: succeeded (pre-existing warnings only — no new warnings)
✅ Tests: 418/418 passed (7 new tests added: 3 for removeAt out-of-range, 3 for updateAt out-of-range)

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