fix(autonat): dial first address in multi-address DialRequest

[akshitj11]

Description

Fixes #6482.

The AutoNAT v2 server selected the last address via addrs.pop() but always reported addr_idx = 0. Selection now uses the first priority-ordered address and reports its index.

Attestation:

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

Notes & open questions

None.

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

[akshitj11]

Updated the selection helper so the chosen address and addr_idx come from the same filtered iteration. Added a regression case where only a nonzero address index is accepted, plus a refusal case when no address is accepted.