DPL: fix rate limiting handling

[ktf]

On success, FairMQ returns a positive number of bytes, not 0.

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
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[ktf]

Tests on hyperloop are fine (Correlations and Histograms).

[ktf]

@shahor02 @davidrohr as discussed.

[alibuild]

Error while checking build/O2/fullCI_slc9 for bba108b at 2025-05-10 04:36:

No log files found

Full log here.

[ktf]

Async reco on the EPN works (and dummy sink is idle) using today + this.

real    43m3.992s
user    1499m15.618s
sys     56m20.009s
Time spent in running the workflow = 2584 s
exitcode = 0
CTFsFilesProcessedMsg = [231252:ctf-reader]: [15:08:22][INFO] CTFReader stops processing, 10 files read, 0 files failed
nCTFsFilesInspected = 10
nCTFsFilesOK = 10
nCTFsFilesFailed = 0
nCTFsProcessed = 433
PVs = 39056
exit code is 0
current size of last AOD file = 3963677919
percentage compared to AOD_FILE_SIZE (= 4000) = 99.00
Max number of parallel AOD mergers will be 8
Checking AO2Ds with un-merged DFs in 001
Checking AO2Ds with un-merged DFs in 002

Merging.