Implement Rx-style auto-detach behavior for observables

[Copilot]

Producers that rely on their returned IDisposable to stop work (e.g. CliWrap cancelling an underlying command) were leaking subscriptions when a consumer callback threw — the upstream subscription was never disposed.

Changes

PowerKit/Observable.cs

• AutoDetachObserver<T> (new, file-scoped): wraps an IObserver<T> and owns a single-assignment source disposable.
  • OnNext — disposes source in catch on throw, rethrows.
  • OnError / OnCompleted — disposes source in finally (success and throw).
  • SetDisposable — atomically assigns the source disposable; if disposal was already flagged (synchronous emission threw before subscribe returned), disposes immediately.
• Observable.Create<T> — now wraps the supplied observer in AutoDetachObserver<T> and assigns the returned disposable via SetDisposable.
• Observable.CreateSynchronized<T> — unchanged; delegates to Create, inheriting auto-detach. Resulting chain: SynchronizedObserver → AutoDetachObserver → original observer.

PowerKit.Tests/ObservableTests.cs

Eight new tests covering:

• OnNext / OnError / OnCompleted throwing — source disposed and exception rethrown.
• Successful OnError / OnCompleted — source disposed.
• Synchronous OnNext during subscribe throws before disposable is returned — disposable disposed immediately once assigned.
• CreateSynchronized — same auto-detach semantics verified.

Example

var observable = Observable.Create<int>(observer =>
{
    // ... start async work ...
    return Disposable.Create(() => CancelWork());  // must run on failure
});

// Previously: if OnNext threw, CancelWork() was never called.
// Now: auto-detach disposes the source subscription, calling CancelWork().
observable.Subscribe(new FakeObserver<int>(
    onNext: _ => throw new InvalidOperationException()
));

[Tyrrrz]

@copilot

[Tyrrrz]

Why not Task.Run instead? @copilot

[Copilot]

Switched to Task.Run in the latest commit. No reason to spin up a dedicated thread here — threadpool is fine for this pattern.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.