Fix StreamOfStreams with atomic CompareExchange

[Aaronontheweb]

Summary

Fixes race condition in FlowPrefixAndTailSpec.PrefixAndTail_must_throw_if_tail_is_attempted_to_be_materialized_twice that was causing intermittent test failures on CI.

Error: Test expected OnError but received OnNext(2) instead.

Root Cause

The issue was a read-then-CAS race condition in SubSource<T>.Logic.SetCallback():

1. Thread 1: reads status (null) → CompareAndSet succeeds → starts data flow
2. Thread 2: reads status (stale null) → CompareAndSet fails → does separate read for retry
3. Race window: Between Thread 2's CAS failure and subsequent read, Thread 1's OnNext(2) flows to wrong subscriber

Solution

1. Added CompareExchange method to AtomicReference<T>: Returns actual previous value instead of just boolean, following standard .NET Interlocked patterns

2. Replaced read-then-CAS with single atomic operation:

   // Before (racy)
   var status = _stage._status.Value; 
   if (status == null) { 
       if (\!_stage._status.CompareAndSet(null, callback)) // Race window here
   }
   
   // After (atomic)
   var previous = _stage._status.CompareExchange(null, callback);
   switch (previous) { ... }

This eliminates all race conditions by using a single atomic operation that both attempts the change AND returns what was actually there.

Testing

• Test now passes consistently (ran 50+ times without failure)
• No other tests affected by AtomicReference changes

Files Changed

• src/core/Akka/Util/AtomicReference.cs - Added CompareExchange method
• src/core/Akka.Streams/Implementation/Fusing/StreamOfStreams.cs - Fixed race condition

Fixes intermittent CI failures from PR #7793.

[Aaronontheweb]

This is the real fix - only read the value once, as the result of the CompareExchange

[Aaronontheweb]

This is, for sure, a bug that may be affecting some other tests too - looks like this was the worst offender though.

[Aaronontheweb]

Made the ordering deterministics

[Aaronontheweb]

This will need a backport to v1.5

[Arkatufus]

LGTM, waiting for CI/CD