tnet: cover udp send error branches

[Rememorio]

Summary

• add direct Linux tests for UDP SendPackets with an empty outbound buffer
• cover dropping a failed queued UDP datagram without returning an error
• cover continuing to send later queued UDP datagrams after a failed packet

CI Alignment

I compared the current tnet workflow with trpc-go. The main red status after #34 is Codecov patch coverage, not a GitHub Actions failure. The tnet push workflow is already green after #33 restricted lint and apidiff to pull requests, so this PR keeps workflow configuration unchanged and focuses on the missing UDP send coverage.

Validation

• go test -run 'TestUDPConnWriteToDropsFailedPacketAndContinues|TestNetFDSendPackets' -v -count=1
• go test -coverprofile=/tmp/tnet_root.cover .
• go test ./...

[WineChord]

LGTM