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>