test: deflake the Gateway waiter lifetime and Bun plugin retention fixtures (#160200)

- server.request-lifetime: the default non-ephemeral test client starts a
  session-history prewarm (`ws:preauth`) alongside `ws:agent.wait`, so the
  "exactly one active root work" assertion raced it (seen as `2 !== 1` in
  CI). The waiter cases now connect as an ephemeral CLI client; failure
  messages name the active holders.
- runtime.retention: under Bun, forced collections inside Promise
  continuations can leave JavaScriptCore's microtask stack holding completed
  async values, so WeakRef checks intermittently saw retained instances.
  Collect inside native setImmediate callbacks instead; assertions unchanged.

src/gateway/server.request-lifetime.test.ts

@@ -6,6 +6,10 @@ import path from "node:path";
 import { setImmediate as nextTurn } from "node:timers/promises";
 import { describe, expect, it, vi } from "vitest";
 import { WebSocket } from "ws";
+import {
+  GATEWAY_CLIENT_IDS,
+  GATEWAY_CLIENT_MODES,
+} from "../../packages/gateway-protocol/src/client-info.js";
 import type { ResponseFrame } from "../../packages/gateway-protocol/src/schema/frames.js";
 import { isAgentRunRestartAbortReason } from "../agents/run-termination.js";
 import { resolveStateDir } from "../config/paths.js";
@@ -14,6 +18,7 @@ import { initializeGlobalHookRunner } from "../plugins/hook-runner-global.js";
 import { createEmptyPluginRegistry } from "../plugins/registry-empty.js";
 import {
   getActiveGatewayRootWorkCount,
+  getActiveGatewayRootWorkHolders,
   markGatewayRestartDraining,
 } from "../process/gateway-work-admission.js";
 import { getAsyncWorkSignal } from "../shared/async-work-scope.js";
@@ -93,7 +98,16 @@ describe("public Gateway close request lifetime", () => {
         await gateway.server.startupSettled;
         phase = "WebSocket connection";
         ws = await gateway.openWs();
-        await connectOk(ws, { scopes: ["operator.admin"] });
+        // An ephemeral CLI avoids admitting history prewarming alongside the waiter.
+        await connectOk(ws, {
+          scopes: ["operator.admin"],
+          client: {
+            id: GATEWAY_CLIENT_IDS.CLI,
+            mode: GATEWAY_CLIENT_MODES.CLI,
+            version: "1.0.0",
+            platform: "test",
+          },
+        });
         const response = onceMessage<Pick<ResponseFrame, "type" | "id" | "ok" | "error">>(
           ws,
           (frame) => frame.type === "res" && frame.id === "wait-for-shutdown",
@@ -114,7 +128,9 @@ describe("public Gateway close request lifetime", () => {
         requestSent = true;
         phase = "waiter admission";
         await Promise.race([entered.promise, prematureReply]);
-        expect(getActiveGatewayRootWorkCount()).toBe(1);
+        expect(getActiveGatewayRootWorkCount(), getActiveGatewayRootWorkHolders().join(", ")).toBe(
+          1,
+        );
         // A failing owner is released through the real terminal registry, not an abandoned wait.
         releaseTimer = setTimeout(() => {
           emergencyRelease = true;

src/plugins/runtime.retention.test-support.ts

@@ -3,6 +3,7 @@ import fs from "node:fs";
 import { createRequire } from "node:module";
 import os from "node:os";
 import path from "node:path";
+import { setImmediate as scheduleImmediate } from "node:timers";
 import { setImmediate } from "node:timers/promises";
 import { withEnvAsync } from "../test-utils/env.js";
 import {
@@ -24,8 +25,13 @@ async function collect() {
   assert.ok(gc, "The retention child requires --expose-gc");
   const control = new WeakRef({ unowned: true });
   for (let pass = 0; pass < 8; pass += 1) {
-    await setImmediate();
-    gc();
+    // JavaScriptCore's promise-microtask stack can retain completed async values.
+    await new Promise<void>((resolve) => {
+      scheduleImmediate(() => {
+        gc();
+        resolve();
+      });
+    });
   }
   assert.equal(control.deref(), undefined, "Unowned control must collect");
 }