test: cover native UDP continuity after closed destinations

[cppla]

Summary

• Add a real native UDP regression for healthy replies before and after attempts to send to a dynamically reserved closed loopback port.
• Require the same PacketConn, relay UDP source, exact payload and source address throughout; no fabricated socket errors or association replacement.
• Keep the destination closed until a later healthy echo completes on the serial request worker; bound receive/fixture cleanup and mark observed port reuse inconclusive.

Validation

• Local make check and make race: passed.
• Linux arm64 in an existing read-only Docker image, isolated network and bounded tmpfs: all TestQUICDatagram* tests repeated 10 times, 120 top-level passes, zero failures/skips.
• Independent test-oracle and scope review: passed.
• Exact head d20d618fc4775859a01e60c4b1bb1e2aecb4ec3d: CI 35809096247 (all 10 jobs), CodeQL 35809096226 and Linux netem 35809096313 succeeded.
• Merged main 39138289ca4b6335d22bb8f49a7d99160dc0429f has the identical tested tree; CI 35809274000 (all 10 jobs, including container integration/OCI), CodeQL 35809274013 and Linux netem 35809273992 succeeded.
• A subsequent review identified Windows bind-errno handling in the test fixture; follow-up test: handle Windows UDP fixture port reuse #27 corrects it and adds targeted Windows runtime CI.

Scope and evidence limits

Test-only: no runtime, protocol, dependency or default changes. This verifies continuity in the exercised environments; it does not establish delivery of ICMP to an unconnected socket or observation of any particular errno. Separate ignored congestion-control experiments are not part of this PR. No release/tag.

[Copilot]

Copilot review overview

🟡 Changes recommended

Add Windows-aware handling for WSAEADDRINUSE during port-reuse detection.

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Review effort: Lite
Findings: 1

Open (1)

• Use Windows-aware address-in-use detection · New

What changed in this PR

Adds a native UDP regression test for association continuity after sends to closed loopback destinations.

Changes:

• Tests healthy UDP replies before and after closed-port attempts.
• Verifies payload, source address, and association continuity.
• Adds bounded cleanup and port-reuse handling.

File	Summary
internal/​tunnel/​datagram_icmp_recovery_test.go	Adds the native UDP continuity regression test.

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