[Repo Assist] Add AsyncSeq.withCancellation (design parity with TaskSeq, #277)

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Summary

Adds AsyncSeq.withCancellation, which returns a new AsyncSeq<'T> that passes a given CancellationToken to GetAsyncEnumerator, overriding whatever token would otherwise be supplied by the caller.

This mirrors TaskSeq.withCancellation and is part of the ongoing design-parity effort tracked in #277.

Motivation

When consuming IAsyncEnumerable<'T> sequences from external libraries (e.g. Entity Framework Core), the library often uses the token passed to GetAsyncEnumerator for cancellation. Previously there was no direct way to inject a specific token into an AsyncSeq pipeline. withCancellation fills this gap.

Implementation

let withCancellation (cancellationToken: CancellationToken) (source: AsyncSeq<'T>) : AsyncSeq<'T> =
    { new IAsyncEnumerable<'T> with
        member _.GetAsyncEnumerator(_ct) = source.GetAsyncEnumerator(cancellationToken) }

This is the same approach as TaskSeq.withCancellation: wrap the source in a new IAsyncEnumerable<'T> that ignores the incoming token and passes the specified one instead. Simple, zero-allocation wrapper (beyond the object allocation for the interface).

Changes

• src/FSharp.Control.AsyncSeq/AsyncSeq.fs — implementation (inside #if !FABLE_COMPILER)
• src/FSharp.Control.AsyncSeq/AsyncSeq.fsi — public signature with XML-doc
• tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs — 4 new tests
• RELEASE_NOTES.md — 4.10.0 entry
• version.props — bumped to 4.10.0

Design Parity Research

As part of researching #277, here is a summary of the broader gap between TaskSeq and AsyncSeq. Items in TaskSeq but missing from AsyncSeq (highest priority first):

Function	Notes
withCancellation	✅ Added in this PR
partition / partitionAsync	Split into two arrays by predicate
compareWith / compareWithAsync	Lexicographic sequence comparison
tryTail	Safe tail returning option
skipWhileInclusive / skipWhileInclusiveAsync	Mirrors existing takeWhileInclusive
takeWhileInclusiveAsync	Async variant of existing takeWhileInclusive
appendSeq / prependSeq	Convenience: append/prepend seq<'T>
collectAsync	Async variant of collect (bind)
delay	Delay sequence creation
lengthBy / lengthByAsync / lengthOrMax	Count with projection / capped count
insertManyAt / removeManyAt	Bulk insert/remove
where / whereAsync	Aliases for filter/filterAsync

Note: AsyncSeq has richer reactive/event-stream features not present in TaskSeq (observables, channels, merge/interleave, combineLatest, parallel map/iter, zapp, cache, etc.) — these are intentional additions that reflect AsyncSeq's broader use cases.

Test Status

✅ Build succeeded (0 errors, pre-existing warnings only — NU1605, FS9999 for groupByAsync, FS0044 for deprecated toAsyncEnum/ofAsyncEnum in existing tests).

✅ All 321 tests pass (dotnet test FSharp.Control.AsyncSeq.sln).

• Fixes We want design parity with FSharp.Control.TaskSeq #277

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