refactor(campaigns): remove dead future-start_date scheduling code

Now that only the manager sets start_date, and only at the moment of
approval (which immediately activates the campaign), a draft campaign
can never have a start_date. This made three things permanently
unreachable: the hourly _run_scheduled_campaign_activation cron job,
the activate_campaign 409/force future-date guard, and a query filter
hiding tests from scheduled-but-inactive draft campaigns. Removes all
three rather than leaving dead code behind. Also adds the missing
manager-cannot-directly-activate router test.

backend/app/services/test_crud_service.py

@@ -22,7 +22,7 @@ from app.models.enums import TestState
 from app.models.technique import Technique
 from app.models.test import Test
 from app.models.test_template import TestTemplate
-from app.models.campaign import Campaign, CampaignTest
+from app.models.campaign import CampaignTest
 from app.models.audit import AuditLog
 from app.utils import escape_like
 
@@ -89,22 +89,6 @@ def list_tests(
         linked = db.query(CampaignTest.test_id).distinct().subquery()
         query = query.filter(~Test.id.in_(linked))
 
-    # Always hide tests from scheduled campaigns that haven't started yet.
-    # A "scheduled-but-not-yet-active" campaign = draft status + start_date in future.
-    now = datetime.utcnow()
-    future_draft_tests = (
-        db.query(CampaignTest.test_id)
-        .join(Campaign, Campaign.id == CampaignTest.campaign_id)
-        .filter(
-            Campaign.status == "draft",
-            Campaign.start_date.isnot(None),
-            Campaign.start_date > now,
-        )
-        .distinct()
-        .subquery()
-    )
-    query = query.filter(~Test.id.in_(future_draft_tests))
-
     # Return query.order_by(Test.created_at.desc()).offset(offset).limit(limit)....
     return query.order_by(Test.created_at.desc()).offset(offset).limit(limit).all()