PR #316 fixed a memory leak when a coroutine awaiting a QFuture is destroyed before the future completes. This PR adds a regression test for that scenario.

Test coverage

• Awaiter lifecycle test (testAwaiterDestroyedBeforeFutureFinishes): Creates a QFuture, starts a coroutine that co_awaits it, destroys the coroutine before the future completes, then completes the future. Verifies:
  • No crash or use-after-free when the QFutureWatcher signal fires on a destroyed coroutine
  • Continuation callback is never invoked (confirming proper cleanup)

The test exercises the fix where QFutureWatcher is parented to the awaiter's dummy QObject, ensuring cleanup when the awaiter is destroyed.

// QPromise controls when the future completes
QPromise<int> promise;
promise.start();

{
    // Coroutine co_awaits the future, creating the awaiter
    auto continuation = [](QFuture<int> f) -> QCoro::Task<int> {
        co_return co_await f;  // WaitForFinishedOperationBase created here
    }(promise.future()).then([&](int) { /* ... */ });
    
    // Awaiter destroyed here when continuation goes out of scope
}

// Future completed after awaiter destruction - no crash
promise.addResult(42);
promise.finish();

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