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

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**:

- [x] I have read every line of this diff, understand what it does, and can explain it in review.

Pull-Request: #6514.

Author: akshitj11

protocols/autonat/CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 0.16.0
 
+- Fix AutoNAT v2 server preserving the selected address index for multi-address `DialRequest`s.
+  See [PR 6514](https://github.com/libp2p/rust-libp2p/pull/6514).
+
 - Use `futures-timer` instead of tokio's timer for stream timeouts so the bounded `Delay` works on
   `wasm32`; tokio's timer has no driver in the browser and panics at runtime.
   See [PR 6488](https://github.com/libp2p/rust-libp2p/pull/6488).

protocols/autonat/src/v2/server/handler/dial_request.rs

@@ -273,8 +273,9 @@ where
         }
     };
     all_addrs.clone_from(&addrs);
-    let idx = 0;
-    let addr = addrs.pop().ok_or(HandleFail::DialRefused)?;
+    let idx = addrs.len().checked_sub(1).ok_or(HandleFail::DialRefused)?;
+    let addr = addrs.remove(idx);
+
     *tested_addrs = Some(addr.clone());
     *data_amount = 0;
     if addr != observed_multiaddr {