test(jetstream): cover bulk-create debounce and flush semantics

Uses @std/testing/time FakeTime to deterministically advance the
500ms debounce. Covers accumulation, timer restart on each push,
the bulk-create verb + payload shape, and per-did buffer
isolation with cleared state after flush.

deno.json

@@ -20,6 +20,7 @@
     "@oak/oak": "jsr:@oak/oak@^17.2.0",
     "@opensearch-project/opensearch": "npm:@opensearch-project/opensearch@^3.6.0",
     "@skyware/jetstream": "npm:@skyware/jetstream@^0.2.5",
-    "@std/assert": "jsr:@std/assert@1"
+    "@std/assert": "jsr:@std/assert@1",
+    "@std/testing": "jsr:@std/testing@1"
   }
 }