test(runtime): keep the new migrate-stored pin inside the test-typecheck ledger

The isSystem pin read res.response.status, which tsc reports as possibly
undefined (TS18048) and grew the shrink-only debt ledger by one. Read it
through optional chaining; an undefined response still fails toBe(200).

Claude-Session: https://claude.ai/code/session_01ADzJtzYTLUfgrRZHxkagkX
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

packages/runtime/src/domains/meta-migrate-stored.test.ts

@@ -147,7 +147,7 @@ describe('POST /meta/_migrate-stored — preview by default (#4327)', () => {
         const res = await make({ migrateStoredMetadata }).handleMetadata(
             '/_migrate-stored', ctx({ isSystem: true }), 'POST', { apply: true },
         );
-        expect(res.response.status).toBe(200);
+        expect(res.response?.status).toBe(200);
         expect(migrateStoredMetadata.mock.calls[0][0]).not.toHaveProperty('actor');
     });