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>

frontend/src/api/tests.ts

@@ -62,6 +62,7 @@ export interface TestListFilters {
   platform?: string;
   created_by?: string;
   pending_validation_side?: "red" | "blue";
+  not_in_any_campaign?: boolean;
   offset?: number;
   limit?: number;
 }
@@ -76,6 +77,7 @@ export async function getTests(filters?: TestListFilters): Promise<Test[]> {
   if (filters?.platform) params.append("platform", filters.platform);
   if (filters?.created_by) params.append("created_by", filters.created_by);
   if (filters?.pending_validation_side) params.append("pending_validation_side", filters.pending_validation_side);
+  if (filters?.not_in_any_campaign) params.append("not_in_any_campaign", "true");
   if (filters?.offset !== undefined) params.append("offset", String(filters.offset));
   if (filters?.limit !== undefined) params.append("limit", String(filters.limit));
 

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>
                 ) : (