Fix iSCSI target cascade: drop unmappable groups instead of failing target

When a portal fails to create (e.g. IP not on system), its ID is absent
from portal_id_map. Previously this caused every target referencing that
portal to be skipped entirely, which then cascaded to target-extent
failures.

Now each target's groups are filtered individually: groups with unmapped
portal or initiator references are dropped with a warning, and the target
is still created with the remaining valid groups. This keeps the target
and all its extent associations intact while clearly indicating which
groups need manual attention.

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

truenas_migrate/migrate.py

@@ -386,21 +386,24 @@ async def _migrate_iscsi_targets(
             summary.iscsi_targets_skipped += 1
             continue
 
-        # Verify all referenced portals and initiators were successfully mapped
-        missing = []
+        # Filter out groups whose portal or initiator could not be mapped (e.g. portal
+        # creation failed).  Warn per dropped group but still create the target — a
+        # target without every portal group is valid and preferable to no target at all.
+        valid_groups = []
         for g in target.get("groups", []):
+            unmapped = []
             if g.get("portal") not in portal_id_map:
-                missing.append(f"portal id={g['portal']}")
+                unmapped.append(f"portal id={g['portal']}")
             if g.get("initiator") not in initiator_id_map:
-                missing.append(f"initiator id={g['initiator']}")
-        if missing:
-            msg = f"iSCSI target {name!r}: cannot remap {', '.join(missing)}"
-            log.error("   %s: %s", _bold_red("SKIP"), msg)
-            summary.iscsi_targets_failed += 1
-            summary.errors.append(msg)
-            continue
+                unmapped.append(f"initiator id={g['initiator']}")
+            if unmapped:
+                log.warning("   %s dropping group with unmapped %s",
+                            _yellow("WARN"), ", ".join(unmapped))
+            else:
+                valid_groups.append(g)
 
-        payload = _iscsi_target_payload(target, portal_id_map, initiator_id_map)
+        payload = _iscsi_target_payload({**target, "groups": valid_groups},
+                                        portal_id_map, initiator_id_map)
         log.debug("   payload: %s", json.dumps(payload))
 
         if dry_run: