fix(campaigns): filter existing-test picker to draft + not in any campaign

Backend: add not_in_any_campaign filter to list_tests (subquery on
CampaignTest) and expose it as a query param on GET /tests.
Frontend: the 'Existing Test' tab now requests only
  state=draft & not_in_any_campaign=true
so tests already linked to any campaign or not in draft state
are never shown.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/tests.py

@@ -90,6 +90,9 @@ def list_tests(
     pending_validation_side: Optional[str] = Query(
         None, description="Filter in_review tests pending validation on 'red' or 'blue' side"
     ),
+    not_in_any_campaign: bool = Query(
+        False, description="Only return tests not linked to any campaign"
+    ),
     offset: int = Query(0, ge=0),
     limit: int = Query(50, ge=1, le=200),
     db: Session = Depends(get_db),
@@ -103,6 +106,7 @@ def list_tests(
         platform=platform,
         created_by=created_by,
         pending_validation_side=pending_validation_side,
+        not_in_any_campaign=not_in_any_campaign,
         offset=offset,
         limit=limit,
     )