fix(backend): sort vitest import in stream-many test (CI biome gate)

Reorder `describe, it, expect` → `describe, expect, it` to satisfy the
Biome organizeImports check that gates the all-services CI pipeline.

services/backend/tests/stream-many.test.ts

@@ -1,4 +1,4 @@
-import { describe, it, expect } from "vitest";
+import { describe, expect, it } from "vitest";
 
 /**
  * Lock the contract that the WS `stream_messages` handler + frontend