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/app/services/campaign_crud_service.py

@@ -957,7 +957,7 @@ def serialize_modification_request(db: Session, request: CampaignModificationReq
         "campaign_name": request.campaign.name if request.campaign else None,
         "requested_by": str(request.requested_by) if request.requested_by else None,
         "action": request.action,
-        "test_id": str(request.test_id),
+        "test_id": str(request.test_id) if request.test_id else None,
         "test_name": test.name if test else None,
         "order_index": request.order_index,
         "phase": request.phase,
@@ -1049,10 +1049,10 @@ def approve_modification_request(
     if request.status != "pending":
         raise BusinessRuleViolation("Only pending modification requests can be approved")
 
-    # Mark the request approved *before* applying the underlying change: a
-    # "remove_test" approval deletes the Test row, which cascades (ON DELETE
-    # CASCADE on test_id) and would otherwise wipe out this very request row
-    # before we get a chance to persist its reviewed/approved fields.
+    # A "remove_test" approval deletes the underlying Test row below, which
+    # nulls this row's test_id (test_id uses ON DELETE SET NULL specifically
+    # so this audit record survives that delete instead of being cascaded
+    # away with it — see CampaignModificationRequest.test_id).
     request.status = "approved"
     request.reviewed_by = reviewer_id
     request.reviewed_at = datetime.utcnow()