[Repo Assist] fix: honor CancellationToken in MoveNextAsync (closes #179)

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Closes #179

Summary

When a CancellationToken is passed to GetAsyncEnumerator(ct), it was stored in the state machine data but never checked during iteration. This meant that infinite or long-running taskSeq sequences could not be cancelled via the enumerator's token — the cancel request was silently ignored.

Root Cause

In TaskSeqBuilder.fs, MoveNextAsync() called data.builder.MoveNext() without first checking whether data.cancellationToken had been cancelled. The token was correctly stored and passed to async bindings (e.g., Async<'T> computations and nested IAsyncEnumerable<'T> sources), but a pure synchronous loop like while true do yield 1 never reached an async checkpoint and therefore never saw the cancellation.

Fix

Added a single ThrowIfCancellationRequested() call at the start of the active iteration path in MoveNextAsync():

else
    let data = this._machine.Data
    // Honor the cancellation token passed to GetAsyncEnumerator (fixes #179).
    // ThrowIfCancellationRequested() is a no-op for CancellationToken.None.
    data.cancellationToken.ThrowIfCancellationRequested()
    data.promiseOfValueOrEnd.Reset()
    ...

ThrowIfCancellationRequested() is a no-op for CancellationToken.None (the common case), so there is no overhead beyond a single volatile boolean read per MoveNextAsync call.

Behaviour

• When the token is cancelled, the next call to MoveNextAsync() throws OperationCanceledException — cooperative, not preemptive.
• A sequence that has already completed (completed = true) returns false normally even if the token is cancelled, since the enumeration is already done.
• All existing behaviour is unchanged for CancellationToken.None.

Tests

Added TaskSeq.CancellationToken.Tests.fs with 8 new tests:

Test	Description
Pre-cancelled token on infinite seq	Throws on first MoveNextAsync
Pre-cancelled token on finite seq	Throws on first MoveNextAsync
Normal iteration (non-cancelled)	All 5 items produced correctly
CancellationToken.None	All 5 items produced correctly
Token cancelled after partial iteration	Throws on next MoveNextAsync
Pre-cancelled token, body not entered	Body doesn't run before cancel
Token cancelled mid-infinite iteration	Throws on next call after cancel
Multiple enumerators, independent tokens	Only cancelled enumerator throws

Test Status

✅ Build succeeded (Release)
✅ All 3858 tests pass (8 new + 3850 existing)
✅ dotnet fantomas . --check passes

Note: Tests were run using SDK 10.0.102 (closest available in this environment); CI will validate against 10.0.103.

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