align deletion test guidance

ai/backend-context.md

@@ -50,6 +50,9 @@ intentionally unclaimed; the built-in health endpoint is `/up`.
   through additional repositories.
 - Test use-case branches at the use-case seam. Do not repeat every branch in
   controller or HTTP tests.
+- Keep deletion behavior tests in the relevant deletion use-case suite. Do
+  not test deletion policy through entity, repository, or database-constraint
+  tests.
 - Plain entities, value objects, use cases, middleware, and controller units
   should extend `PHPUnit\Framework\TestCase` when they do not need Laravel.
 - Extend `Tests\TestCase` only when a test needs Laravel's container, facades,