Fixes #20466: Correct handling of assigned filter logic

[pheus]

Fixes: #20466

Summary

• Correct the custom GraphQL filter IPAddressFilter.assigned to use the nested relation prefix supplied by Strawberry‑Django.
• Prior behavior built a non‑prefixed Q, so when used under another filter (e.g., Device → primary_ip4), Django attempted to resolve assigned_object_id on the outer model and failed.
• New behavior dynamically qualifies the lookup with prefix, ensuring the condition targets the nested IPAddress.

Change

@strawberry_django.filter_field()
def assigned(self, value: bool, prefix) -> Q:
-    return Q(assigned_object_id__isnull=(not value))
+    return Q(**{f"{prefix}assigned_object_id__isnull": not value})

Why

Nested GraphQL queries like the example below errored due to the missing prefix; with this change they work as intended.

query {
  device_list(filters: { primary_ip4: { assigned: true } }) {
    id
    name
    primary_ip4 { address }
  }
}

Follow‑up

• Propose a small audit for other @strawberry_django.filter_field methods that construct raw Q(...) lookups without applying prefix.