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.

frontend/src/components/AddTestToCampaignModal.tsx

@@ -106,7 +106,7 @@ export default function AddTestToCampaignModal({
 
   const { data: allTests, isLoading: testsLoading } = useQuery({
     queryKey: ["tests", "for-campaign-picker"],
-    queryFn: () => getTests({ limit: 200 }),
+    queryFn: () => getTests({ state: "draft", not_in_any_campaign: true, limit: 200 }),
     enabled: open && tab === "existing",
   });
 
@@ -506,7 +506,7 @@ export default function AddTestToCampaignModal({
                     <p className="text-sm">
                       {existingSearch
                         ? "No tests match your search."
-                        : "No available tests to add."}
+                        : "No draft tests available. All existing tests are already in a campaign."}
                     </p>
                   </div>
                 ) : (