fix(xfer): Prevent no-wait early completion race

[leumor]

Summary

• fix a race in BulkTransmitter no-wait completion logic where send() could complete before the transmitter observed a newly received block
• replace prb.hasWholeFile() early-complete gating with transmitter-local block observation tracking
• only complete no-wait mode after this transmitter has observed all blocks and has no pending local blocks left to queue
• add a deterministic regression test that forces the ordering window (hasWholeFile=true before blockReceived() notification) and verifies no premature completion

How To Test

• ./gradlew test --tests '*BulkTransmitterTest'
• ./gradlew test --tests '*BulkTransmitterTest.send_whenNewBlockArrivesWithPacketsInFlight_expectOpportunisticSecondSend' --rerun-tasks

Validation Notes

• stress-ran the previously failing test method 8 times with --rerun-tasks; all runs passed
• this specifically targets intermittent failures seen during Ubuntu amd64 packaging builds

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
81.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud