Fix DispatchStreamProcessorTest.raceToStream flakiness

[Copilot]

raceToStream failed intermittently with dsp has streamers? expected false was true after cancel. Async subscribe/cancel could leave streamers attached past the wait, and the await helpers used unreliable loop-based timeouts.

DispatchStreamProcessor.stream()

• Add the streamer to the array first, then register with cancellation (PublishProcessor pattern)
• If dispose raced the add, remove again so a disposed streamer cannot stick in the array

Test helpers (StreamableBaseTest)

• Drive waits off wall-clock System.nanoTime() / TimeUnit instead of loop counters (avoids excess delay and early timeout)
• Add assertNoStreamers / assertHasStreamers (with optional count)

DispatchStreamProcessorTest

• Wait for both subscribers before cancel in raceToStream:

assertHasStreamers(dsp, 1000, 2);
ts.cancel();
ts2.cancel();
assertNoStreamers(dsp, 1000);

• Replace bare assertFalse(hasStreamers()) / await* pairs with the assert helpers elsewhere in the class

• Fixes DispatchStreamProcessorTest unit test failure #8274

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (f6d80d2) to head (7b65a64).
⚠️ Report is 2 commits behind head on 4.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8275      +/-   ##
============================================
+ Coverage     98.58%   98.61%   +0.03%     
+ Complexity     7147     7146       -1     
============================================
  Files           824      824              
  Lines         50782    50784       +2     
  Branches       6906     6907       +1     
============================================
+ Hits          50064    50083      +19     
+ Misses          488      481       -7     
+ Partials        230      220      -10     

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