[Auto Maintainer Assistant] Add AsyncSeq.chunkBy and AsyncSeq.chunkByAsync

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Closes #156

Summary

Adds two new functions to the AsyncSeq module that group consecutive elements sharing the same key — analogous to Seq.groupBy but for runs of adjacent equal keys rather than all elements:

• AsyncSeq.chunkByAsync — accepts an async key projection 'T -> Async<'Key>
• AsyncSeq.chunkBy — accepts a synchronous key projection 'T -> 'Key

Both return AsyncSeq<'Key * 'T list>. The return type uses 'T list (rather than ResizeArray) to avoid leaking implementation details.

Example

asyncSeq { yield 1; yield 1; yield 2; yield 2; yield 1 }
|> AsyncSeq.chunkBy id
// yields: (1, [1;1]), (2, [2;2]), (1, [1])

This mirrors the Seq.chunkBy function from FSharpPlus described in the issue.

Implementation Notes

• chunkBy delegates to chunkByAsync via projection >> async.Return, following the same pattern as groupBy/groupByAsync.
• The implementation uses the same GetEnumerator/MoveNext pattern as chunkBySize.
• 'Key : equality constraint is required and is declared in the .fsi signatures.

Test Status

• Debug build passes (dotnet build -c Debug)
• Release build passes (dotnet build -c Release)
• Debug tests pass (dotnet test -c Debug) — 187/187
• Release tests pass (dotnet test -c Release) — 187/187

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