refactor(campaigns): fix approve() error message, add terminal-state test

backend/app/domain/entities/campaign.py

@@ -161,7 +161,7 @@ class CampaignEntity:
         if self.status != CampaignStatus.pending_approval:
             raise InvalidStateTransition(
                 self.status.value, CampaignStatus.active.value,
-                [s.value for s in VALID_TRANSITIONS[self.status]],
+                [CampaignStatus.pending_approval.value],
             )
         self.status = CampaignStatus.active
 

backend/tests/test_campaign_entity.py

@@ -212,6 +212,12 @@ def test_approve_from_draft_raises():
         e.approve()
 
 
+def test_approve_from_completed_raises():
+    e = _entity("completed", test_count=1)
+    with pytest.raises(InvalidStateTransition):
+        e.approve()
+
+
 # ── 11. Reject ─────────────────────────────────────────────────────────