test: ipv6 end-to-end scenarios

vm01 gains a primary IPv6 next to its IPv4, one standalone IPv6 address
joins the dataset; covers ip_version on both sources, combination with
filters, the IPv6-preferring behaviour of netbox's combined primary IP,
and the invalid value error path.

e2e/conftest.py

@@ -59,6 +59,7 @@ def run_inventory(stack):
         source: str = "",
         filter_name: str = "",
         filter_value: str = "",
+        ip_version: str = "",
         args: tuple[str, ...] = ("--list",),
         token: str | None = None,
     ):
@@ -71,6 +72,7 @@ def run_inventory(stack):
             "NETBOX_INVENTORY_SOURCE": source or "ip-addresses",
             "NETBOX_INVENTORY_FILTER": filter_name,
             "NETBOX_INVENTORY_FILTER_VALUE": filter_value,
+            "NETBOX_INVENTORY_IP_VERSION": ip_version or "both",
         }
         result = subprocess.run(
             [SCRIPT, *args], capture_output=True, text=True, timeout=120, env=env