Extend IPv6 STUN connect error handling to cover more unreachable cases

In addition to ENETUNREACH, also treat HostUnreachable, Unsupported
and NetworkDown as non-fatal when probing IPv6 — all indicate the
family is unavailable. Guard is limited to IpFamily::V6 so any IPv4
connect failure still propagates as an error.

src/network/stun.rs

@@ -52,7 +52,13 @@ pub async fn stun_probe_family(stun_addr: &str, family: IpFamily) -> Result<Opti
     if let Some(addr) = target_addr {
         match socket.connect(addr).await {
             Ok(()) => {}
-            Err(e) if e.kind() == std::io::ErrorKind::NetworkUnreachable => return Ok(None),
+            Err(e) if family == IpFamily::V6 && matches!(
+                e.kind(),
+                std::io::ErrorKind::NetworkUnreachable
+                | std::io::ErrorKind::HostUnreachable
+                | std::io::ErrorKind::Unsupported
+                | std::io::ErrorKind::NetworkDown
+            ) => return Ok(None),
             Err(e) => return Err(ProxyError::Proxy(format!("STUN connect failed: {e}"))),
         }
     } else {