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/services/test_crud_service.py

@@ -19,6 +19,7 @@ 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 CampaignTest
 from app.models.audit import AuditLog
 from app.utils import escape_like
 
@@ -31,6 +32,7 @@ def list_tests(
     platform: str | None = None,
     created_by: uuid.UUID | None = None,
     pending_validation_side: str | None = None,
+    not_in_any_campaign: bool = False,
     offset: int = 0,
     limit: int = 50,
 ) -> list[Test]:
@@ -55,6 +57,9 @@ def list_tests(
             Test.state == TestState.in_review,
             Test.blue_validation_status.in_(["pending", None]),
         )
+    if not_in_any_campaign:
+        linked = db.query(CampaignTest.test_id).distinct().subquery()
+        query = query.filter(~Test.id.in_(linked))
 
     return query.order_by(Test.created_at.desc()).offset(offset).limit(limit).all()