Detect incomplete IPUDPTransfer datagrams

[danielsyauqi]

Summary

• Reject datagrams whose received length differs from the generated payload length.
• Exit with status 1 when a benchmark transfer fails.
• Add a regression test for the 1,472-byte success and 1,473-byte failure boundary.

Why

IPUDPTransfer previously treated any non-nil read as a completed transfer. With a 1,473-byte payload, the receiver gets an empty datagram, but the benchmark reported success. Checking length catches empty and truncated receives without adding a byte-by-byte comparison to the timed packet loop.

Testing

• swift test -c release
• swift test --filter IPUDPTransferTests -Xswiftc -warnings-as-errors
• swift format lint --strict Package.swift Sources/Tools/IPUDPTransfer/main.swift Tests/IPUDPTransferTests/IPUDPTransferTests.swift
• Release executable: 1,472-byte payload exits 0; 1,473-byte payload exits 1; three iterations of 300 packets at 1,000 bytes exit 0.

Closes #250

[agnosticdev]

Thank you for the change. Just one comment regarding the test.