fix(testing): isolate reminder event continuations - #10899

This PR addresses a flaky reminders test by ensuring that waits on reminder diagnostic events do not resume continuations inline on the diagnostic event emitter thread, aligning reminder diagnostic wait behavior with other lifecycle-state waiters.

Changes:

• Replace Rx FirstAsync(...).ToTask(...) conversions with a TaskCompletionSource-based wait using RunContinuationsAsynchronously.
• Ensure cancellation and error propagation disposes subscriptions and completes the returned task appropriately.
• Add a regression test asserting that an ExecuteSynchronously continuation attached to WaitForReminderServiceStartedAsync does not run inline.

Previously missed (1) — in code that hasn't changed since the last review.

test/Orleans.Testing.Reminders/ReminderDiagnosticObserver.cs:335

• WaitForEventAsync calls predicate(value) inside the subscription callback without handling exceptions. If the predicate throws, the exception will escape the Rx OnNext delegate (potentially crashing the event producer) and the returned task will never complete or clean up its registration/subscription. This is a semantic regression compared to FirstAsync(predicate) which faults the sequence when the predicate throws.

            value =>
            {
                if (!predicate(value))
                {
                    return;

Previously missed (1) — in code that hasn't changed since the last review.

test/Orleans.Reminders.Tests/Diagnostics/ReminderEventsTests.cs:58

• The assertion Assert.NotEqual(emitterThread, continuationThread) is currently comparing the continuation thread against the test thread (the one calling EmitReminderServiceStarted). TaskCreationOptions.RunContinuationsAsynchronously guarantees the continuation is not invoked inline on completion, but it does not guarantee it will run on a different managed thread than the caller (a ThreadPool thread can dequeue its own queued work later). This can make the regression test flaky and fail even when continuations are correctly asynchronous.

A more robust regression is to emit the event from a dedicated non-ThreadPool Thread and assert the continuation does not run on that emitter thread.

        ReminderEvents.EmitReminderServiceStarted(siloAddress);

        Assert.True(continuationRan.Wait(TimeSpan.FromSeconds(5), TestContext.Current.CancellationToken));
        Assert.NotEqual(emitterThread, continuationThread);
    }