[multistage] Fix flaky RAW_MESSAGES assertion in GrpcSenderBackpressureTest

[gortiz]

Description

GrpcSenderBackpressureTest.senderObservesBackpressureFromSlowReceiver is flaky. It asserted an exact relationship between the send loop's sendCount and the RAW_MESSAGES stat:

assertTrue(rawMessages == sendCount || rawMessages == sendCount + 1, ...);

That equality does not hold in general. sendCount counts every send() call the loop makes, but RAW_MESSAGES is only incremented once a block passes the terminated / early-terminated guard inside GrpcSendingMailbox.sendInternal — and SendingMailbox.send(MseBlock.Data) returns void, so the loop cannot tell an aborted attempt from a real send. When the tail of the run interleaves with the watchdog's cancel(), one or more attempts bump sendCount without incrementing RAW_MESSAGES, so RAW_MESSAGES legitimately trails sendCount. CI hit rawMessages == sendCount - 1, which the assertion rejected.

Fix

Replace the exact check with two bounds that hold regardless of how the tail interleaves with the cancel:

• RAW_MESSAGES <= sendCount + 1 — each send() pushes at most one block through processAndSend (one RAW_MESSAGES increment), and the cancel adds at most one error EOS.
• RAW_MESSAGES >= polledCount — the receiver cannot poll a block the sender never pushed. polledCount is measured independently on the receiver side, so this still guards against RAW_MESSAGES not being tracked at all.

Test-only change; no production code is touched.

Testing

mvn -pl pinot-query-runtime -Dtest=GrpcSenderBackpressureTest test passes; spotless / checkstyle / license checks pass on the module.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.37%. Comparing base (2892236) to head (cee3be6).

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #19020      +/-   ##
============================================
- Coverage     65.39%   65.37%   -0.02%     
  Complexity     1405     1405              
============================================
  Files          3423     3423              
  Lines        215968   215968              
  Branches      34186    34186              
============================================
- Hits         141228   141185      -43     
- Misses        63380    63418      +38     
- Partials      11360    11365       +5     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-25	65.37% <ø> (-0.02%)	⬇️
temurin	65.37% <ø> (-0.02%)	⬇️
unittests	65.37% <ø> (-0.02%)	⬇️
unittests1	57.09% <ø> (-0.03%)	⬇️
unittests2	37.56% <ø> (-0.02%)	⬇️

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