Fix crash when cancelling subscription for custom PersistenceKey

[pyrtsa]

We have a custom conformance of PersistenceKey that called Shared<Value>.Subscription.cancel() on the base persistence key in its own onCancel closure:

struct PersistenceKeyProjection<Value: Sendable, Base: PersistenceReaderKey>: PersistenceReaderKey {
  let base: Base

  // ...

  func subscribe(
    initialValue: Value?,
    didSet: @escaping @Sendable (_ newValue: Value?) -> Void
  ) -> Shared<Value>.Subscription {
    let subscription = self.base.subscribe( /* ... */ )
    return Shared.Subscription {
      subscription.cancel()  // ← ⚠️
    }
  }
}

When upgrading to 1.16.0, that logic started crashing with AppStorageKey because Shared<Value>.Subscription also calls cancel() from its deinit and key-value observing does not allow that:

Caution

Cannot remove an observer <_TtCGV22ComposableArchitecture13AppStorageKeyGSqSS__P10$1353d4f8c8Observer 0x6000002848a0> for the key path "stringKey" from <NSUserDefaults 0x600000dd3cf0> because it is not registered as an observer.

In our case, the sufficient workaround was to change the call to subscription.cancel() into just an extended lifetime _ = subscription and call it a day.

But I think it's still a wart that Shared<Value>.Subscription allowed this to crash!

This PR reproduces the crash with a (rather artificial) unit test and follows up with a proposed fix.

[stephencelis]

Thanks! We noticed that Shared.Subscription had this issue and also isn't Sendable a day or two ago. We'll merge this and add sendability soon.