Fix identity cache miss during lookup response verification

Guard discovery triggers in handle_path_broken() and
handle_coords_required() with has_cached_identity() check. When the
XK responder receives an error signal before msg3 completes, the
initiator's identity is unknown, making LookupResponse proof
verification impossible. Skip discovery in this case — the handshake
retry mechanism handles recovery.

Downgrade the identity_cache miss log from ERROR to WARN since it's a
known race condition, not a bug.

src/node/tests/discovery.rs

@@ -262,7 +262,7 @@ async fn test_response_proof_verification_failure() {
 #[tokio::test]
 async fn test_response_identity_cache_miss() {
     // Verify that a response is discarded when the origin lacks the
-    // target's pubkey in identity_cache (should never happen in practice).
+    // target's pubkey in identity_cache (e.g., XK responder before msg3).
     let mut node = make_node();
     let from = make_node_addr(0xAA);