Fix KeyError: port in portal IP conflict detection

_ip_set() used (ip, port) tuples for conflict matching. Since port
is no longer present in listen entries, key on IP only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

truenas_migrate/migrate.py

@@ -316,7 +316,7 @@ async def _migrate_iscsi_portals(
         log.error(msg); summary.errors.append(msg); return
 
     def _ip_set(p: dict) -> frozenset:
-        return frozenset((l["ip"], l["port"]) for l in p.get("listen", []))
+        return frozenset(l["ip"] for l in p.get("listen", []))
 
     existing_ip_sets = [(_ip_set(p), p["id"]) for p in existing]
     log.info("  Destination has %d existing portal(s).", len(existing))