fix(campaigns): null-safe test_id serialization, correct stale comments

serialize_modification_request crashed toward a string 'None' instead of
JSON null for requests whose test_id was nulled by the SET NULL cascade.
Also corrects two comments that still described the debunked ordering
theory instead of the actual SET NULL fix, and fixes a test that
asserted post-cascade state without committing first (the ORM only
picks up out-of-band DB-side SET NULL on already-loaded objects after
expire_on_commit forces a re-read, matching real router behavior).

backend/tests/test_campaign_approval.py

@@ -177,6 +177,7 @@ from app.services.campaign_crud_service import (
     approve_modification_request,
     reject_modification_request,
     list_modification_requests,
+    serialize_modification_request,
 )
 
 
@@ -278,14 +279,18 @@ class TestModificationRequests:
         request_id = request.id
 
         approve_modification_request(db, str(request_id), reviewer_id=admin_user.id)
+        db.commit()
 
         cts = db.query(CampaignTest).filter(CampaignTest.campaign_id == active_campaign_with_test.id).all()
         assert len(cts) == 0
 
-        # The request row must survive the cascade-delete of the underlying Test
-        # (ondelete="CASCADE" on CampaignModificationRequest.test_id) — this locks
-        # in the fix where the request's status is persisted BEFORE the test is
-        # removed, not after.
+        # The request row must survive deletion of the underlying Test — this
+        # locks in test_id's ondelete="SET NULL" (not CASCADE), which is what
+        # keeps this audit record intact after the test it referenced is gone.
+        # (Commit above first: ON DELETE SET NULL is applied DB-side by the
+        # cascade, and the ORM only picks that up on already-loaded objects
+        # after expire_on_commit forces a fresh read — exactly what happens
+        # for real in the router, which commits before serializing.)
         reloaded = (
             db.query(CampaignModificationRequest)
             .filter(CampaignModificationRequest.id == request_id)
@@ -296,6 +301,13 @@ class TestModificationRequests:
         assert reloaded.reviewed_by == admin_user.id
         assert reloaded.reviewed_at is not None
 
+        # Serialization must not crash and must emit a real null, not the
+        # string "None", for a request whose test_id has been nulled out.
+        assert reloaded.test_id is None
+        serialized = serialize_modification_request(db, reloaded)
+        assert serialized["test_id"] is None
+        assert serialized["test_name"] is None
+
     def test_approve_already_decided_request_raises(self, db, active_campaign_with_test, admin_user):
         ct = db.query(CampaignTest).filter(CampaignTest.campaign_id == active_campaign_with_test.id).first()
         request = create_modification_request(