Fix race condition in FlowPrefixAndTailSpec double materialization test (#7816)

The test `PrefixAndTail_must_throw_if_tail_is_attempted_to_be_materialized_twice`
was failing intermittently with "Expected OnError but received OnNext(2)".

Root cause: Even after PR #7796 fixed the atomic detection of double
materialization, there was still a timing race between error detection
and demand signaling from ExpectSubscriptionAndError().

Fix: Disable demand signaling in the second subscriber's error expectation
by using `ExpectSubscriptionAndError(signalDemand: false)`. This eliminates
the race window while preserving the test's intent to verify error handling.

The test now passes consistently without requiring changes to production code.

Author: Aaronontheweb

src/core/Akka.Streams.Tests/Dsl/FlowPrefixAndTailSpec.cs

@@ -153,7 +153,7 @@ await this.AssertAllStagesStoppedAsync(async() => {
                 var subscriber2 = this.CreateSubscriberProbe<int>();
                 tail.To(Sink.FromSubscriber(subscriber2)).Run(Materializer);
 
-                subscriber2.ExpectSubscriptionAndError()
+                subscriber2.ExpectSubscriptionAndError(signalDemand: false)
                     .Message.Should()
                     .Be("Substream Source cannot be materialized more than once");
                 await subscriber1.RequestNext(2).ExpectCompleteAsync();