test: wait on the gap marker rather than a depth that never moves

The refused-delivery test waited for `depth(subId)` to reach 1 before
claiming the marker, but `discard` removes the entry and appends the
marker in its place, so the depth is 1 before, during and after. The
wait returned at once, and the claim came back `inflight` — the refused
delivery still held the lease it was invoked on — leaving the assertion
reading `undefined`. It only passed on the slack in `invoked`'s poll
interval, which a loaded runner takes away.

Waiting on the claim itself is the barrier the depth was standing in
for. Dropping `answer = 200` with it: `beforeEach` already resets it,
and a redelivery settling before the last assertion would clear the
failure count it reads.

src/backend/services/events/workerInvoker.integration.test.ts

@@ -556,15 +556,16 @@ describe('what each answer does to the delivery', () => {
 
         // The event is gone and a marker stands in its place, so the
         // subscription learns there was one rather than reading silence.
+        // Waited on by claiming it: depth is 1 across the swap either way.
         await waitUntil(async () =>
-            expect(await pending().depth(subId)).toBe(1),
+            expect(
+                (await pending().claim(subId, { leaseMs: 0 }))?.event,
+            ).toMatchObject({
+                op: 'gap',
+                reason: 'handler_rejected',
+            }),
         );
-        answer = 200;
-        const claimed = await pending().claim(subId, { leaseMs: 0 });
-        expect(claimed?.event).toMatchObject({
-            op: 'gap',
-            reason: 'handler_rejected',
-        });
+        expect(await pending().depth(subId)).toBe(1);
         // It counted: a refusal is still a handler that did not work.
         await expect(
             env.server.clients.redis.get(`ev:qf:{${subId}}`),