fix: stabilize stale timeout tests with fixed Date.now()

Tests 'should use default timeout when config not provided' (manager.test.ts)
and 'should use DEFAULT_MESSAGE_STALENESS_TIMEOUT_MS when not configured'
(task-poller.test.ts) failed in CI because Date.now() drifted between
test setup (when creating timestamps like Date.now() - 46*60*1000) and
actual execution inside checkAndInterruptStaleTasks().

On slower CI machines, this drift pushed borderline values across
the threshold, causing tasks that should be stale to remain 'running'.

Fix: Mock Date.now with spyOn to return a fixed time, ensuring
consistent timeout calculations regardless of execution speed.

src/features/background-agent/manager.test.ts

@@ -1,5 +1,5 @@
 declare const require: (name: string) => any
-const { describe, test, expect, beforeEach, afterEach } = require("bun:test")
+const { describe, test, expect, beforeEach, afterEach, spyOn } = require("bun:test")
 import { tmpdir } from "node:os"
 import type { PluginInput } from "@opencode-ai/plugin"
 import type { BackgroundTask, ResumeInput } from "./types"
@@ -2781,6 +2781,18 @@ describe("BackgroundManager - Non-blocking Queue Integration", () => {
 })
 
 describe("BackgroundManager.checkAndInterruptStaleTasks", () => {
+  const originalDateNow = Date.now
+  let fixedTime: number
+
+  beforeEach(() => {
+    fixedTime = Date.now()
+    spyOn(globalThis.Date, "now").mockReturnValue(fixedTime)
+  })
+
+  afterEach(() => {
+    Date.now = originalDateNow
+  })
+
    test("should NOT interrupt task running less than 30 seconds (min runtime guard)", async () => {
      const client = {
        session: {

src/features/background-agent/task-poller.test.ts

@@ -1,5 +1,5 @@
 declare const require: (name: string) => any
-const { describe, it, expect, mock } = require("bun:test")
+const { describe, it, expect, mock, spyOn, beforeEach, afterEach } = require("bun:test")
 
 import { checkAndInterruptStaleTasks, pruneStaleTasksAndNotifications } from "./task-poller"
 import type { BackgroundTask } from "./types"
@@ -29,6 +29,18 @@ describe("checkAndInterruptStaleTasks", () => {
       ...overrides,
     }
   }
+  const originalDateNow = Date.now
+  let fixedTime: number
+
+  beforeEach(() => {
+    fixedTime = Date.now()
+    spyOn(globalThis.Date, "now").mockReturnValue(fixedTime)
+  })
+
+  afterEach(() => {
+    Date.now = originalDateNow
+  })
+
 
   it("should interrupt tasks with lastUpdate exceeding stale timeout", async () => {
     //#given