fix: one lapsed membership to a deleted creator no longer fails the Patreon roster

The first live sweep failed with "Patreon member resource has no campaign relationship". The operator ran FC's exact /api/members request in a browser and shared the response.
- It has 104 rows. The browser's settings page shows 43, because FC sends no membership-type filter and so also gets lapses back to 2016.
- Exactly one row has no `campaign` key at all: a former_patron whose membership ended in 2017. Its included reward has no campaign link either, so the creator's page is gone.
- Every other row, including the 4 is_active=false campaigns, has a campaign.

_membership returns None for a campaign-less row only when has_paid_access says it is definitely not paying (a known lapsed status, or a free member), and iter_memberships skips it. That changes no conclusion. A lapsed membership and an absent one both mean "not paying", and no Source can match a campaign with no id.

An active or unrecognised membership without a campaign still raises, because dropping a membership that might be paid would read downstream as a cancellation.

Paging still counts the rows the server sent, not the rows kept. A test pins that, so a skip can't re-read an offset or stop a page short.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

tests/test_patreon_memberships.py

@@ -239,14 +239,72 @@ def test_a_missing_data_list_is_drift(client, payload):
         list(client.iter_memberships(user_id="1"))
 
 
-def test_a_member_with_no_campaign_is_drift(client, payload):
+def _index_of(payload, status):
+    return next(
+        i for i, m in enumerate(payload["data"])
+        if m["attributes"]["patron_status"] == status
+    )
+
+
+@pytest.mark.parametrize("shape", ["null", "absent"])
+def test_an_active_member_with_no_campaign_is_drift(client, payload, shape):
+    """A PAID membership FC cannot attribute must refuse the roster. Dropping it
+    would read downstream as the operator having cancelled it."""
     mangled = json.loads(json.dumps(payload))
-    mangled["data"][0]["relationships"]["campaign"] = {"data": None}
+    rels = mangled["data"][_index_of(mangled, "active_patron")]["relationships"]
+    if shape == "null":
+        rels["campaign"] = {"data": None}
+    else:
+        del rels["campaign"]
     client._request = lambda *a, **k: mangled
     with pytest.raises(PatreonDriftError, match="campaign"):
         list(client.iter_memberships(user_id="1"))
 
 
+def test_an_unrecognised_status_with_no_campaign_is_drift(client, payload):
+    """Only a status KNOWN to mean "not paying" may be skipped. An unknown word
+    could be a paid membership (has_paid_access returns None for it)."""
+    mangled = json.loads(json.dumps(payload))
+    row = mangled["data"][_index_of(mangled, "active_patron")]
+    row["attributes"]["patron_status"] = "some_new_status"
+    del row["relationships"]["campaign"]
+    client._request = lambda *a, **k: mangled
+    with pytest.raises(PatreonDriftError, match="campaign"):
+        list(client.iter_memberships(user_id="1"))
+
+
+def test_a_lapsed_member_whose_creator_is_gone_is_skipped(client, payload):
+    """The live roster's shape (note #3886, CORRECTION 3). A membership that
+    lapsed in 2017 came back with no `campaign` key at all, because the creator's
+    page no longer exists. It used to fail the whole roster. Now it is the only
+    row missing, and every other row still arrives."""
+    mangled = json.loads(json.dumps(payload))
+    lapsed = _index_of(mangled, "former_patron")
+    del mangled["data"][lapsed]["relationships"]["campaign"]
+    client._request = lambda *a, **k: mangled
+    rows = list(client.iter_memberships(user_id="1"))
+    assert len(rows) == len(payload["data"]) - 1
+    assert all(r.campaign_id for r in rows)
+    assert all(r.status == "active_patron" for r in rows)
+
+
+def test_skipping_does_not_end_paging_early(client, payload):
+    """Paging counts the rows the SERVER sent, not the ones kept. Counting kept
+    rows would re-request an offset that was already read, or stop one page
+    short, whenever a row is skipped."""
+    first = json.loads(json.dumps(payload))
+    del first["data"][_index_of(first, "former_patron")]["relationships"]["campaign"]
+    total = 2 * len(payload["data"])
+    first["meta"]["pagination"]["total"] = total
+    second = json.loads(json.dumps(payload))
+    second["meta"]["pagination"]["total"] = total
+    calls = _serve(client, first, second)
+    rows = list(client.iter_memberships(user_id="1"))
+    assert len(calls) == 2
+    assert calls[1][1]["page[offset]"] == str(len(payload["data"]))
+    assert len(rows) == total - 1
+
+
 def test_a_member_with_no_patron_status_is_drift(client, payload):
     mangled = json.loads(json.dumps(payload))
     del mangled["data"][0]["attributes"]["patron_status"]