fix(client): preserve compatible background service (#36583)

packages/client/test/fixture/service.ts

@@ -0,0 +1,39 @@
+import { rename, writeFile } from "node:fs/promises"
+
+const [registration, mode] = process.argv.slice(2)
+if (registration === undefined || mode === undefined) throw new Error("Missing service fixture arguments")
+
+let requests = 0
+const server = Bun.serve({
+  port: 0,
+  async fetch(request) {
+    if (new URL(request.url).pathname !== "/api/health") return new Response(null, { status: 404 })
+    requests += 1
+    if (mode === "modern" && requests === 1) {
+      await writeFile(registration + ".first-request", "")
+      while (!(await Bun.file(registration + ".release").exists())) await Bun.sleep(5)
+      return new Response(null, { status: 503 })
+    }
+    if (mode === "legacy") return Response.json({ healthy: true })
+    return Response.json({ healthy: true, version: "test", pid: process.pid })
+  },
+})
+
+await writeFile(
+  registration + ".tmp",
+  JSON.stringify({
+    id: crypto.randomUUID(),
+    version: mode === "legacy" ? undefined : "test",
+    url: server.url.toString(),
+    pid: process.pid,
+  }),
+  { mode: 0o600 },
+)
+await rename(registration + ".tmp", registration)
+
+const shutdown = () => {
+  server.stop(true)
+  process.exit()
+}
+process.on("SIGTERM", shutdown)
+process.on("SIGINT", shutdown)