fix(autonat): close dial-back connections to prevent memory leak

[imabdulbasit]

Description

The AutoNAT server (v1 and v2) dials clients back on a newly allocated port to probe reachability but never closes the resulting connection: the result is delivered over the client's original connection, while other behaviours keep the dial-back connection alive indefinitely. On QUIC each such connection pins a dedicated quinn::Endpoint (its own UDP socket plus several MiB of buffers), so a probe-serving node leaks memory slowly but without bound. This PR closes the dial-back connection once the probe completes

AI Assistance Disclosure

Tools used (required — write none if no AI was used): Claude Code

Attestation (required):

• I have read every line of this diff, understand what it does, and can explain it in review.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates