fix(background-agent): await stale task aborts before poller exits

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -16,6 +16,20 @@ describe("checkAndInterruptStaleTasks", () => {
   }
   const mockNotify = mock(() => Promise.resolve())
 
+  function createDeferredPromise(): {
+    promise: Promise<void>
+    resolve: () => void
+  } {
+    let resolvePromise = () => {}
+    const promise = new Promise<void>((resolve) => {
+      resolvePromise = resolve
+    })
+    return {
+      promise,
+      resolve: resolvePromise,
+    }
+  }
+
   function createRunningTask(overrides: Partial<BackgroundTask> = {}): BackgroundTask {
     return {
       id: "task-1",
@@ -114,6 +128,39 @@ describe("checkAndInterruptStaleTasks", () => {
     expect(task.error).toContain("no activity")
   })
 
+  it("should await abort before resolving for no-progress stale interruption", async () => {
+    //#given
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 15 * 60 * 1000),
+      progress: undefined,
+    })
+    const deferred = createDeferredPromise()
+    mockClient.session.abort.mockImplementationOnce(() => deferred.promise)
+
+    //#when
+    const interruptPromise = checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { messageStalenessTimeoutMs: 600_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+    })
+    let settled = false
+    void interruptPromise.then(() => {
+      settled = true
+    })
+
+    await Promise.resolve()
+
+    //#then
+    expect(settled).toBe(false)
+
+    deferred.resolve()
+    await interruptPromise
+
+    expect(settled).toBe(true)
+  })
+
   it("should NOT interrupt tasks with NO progress.lastUpdate that are within messageStalenessTimeoutMs", async () => {
     //#given — task started 5 minutes ago, default timeout is 10 minutes
     const task = createRunningTask({
@@ -407,6 +454,45 @@ describe("checkAndInterruptStaleTasks", () => {
     expect(task.error).toContain("session gone from status registry")
   })
 
+  it("should await abort before resolving for session-gone interruption", async () => {
+    //#given
+    const task = createRunningTask({
+      startedAt: new Date(Date.now() - 300_000),
+      progress: {
+        toolCalls: 1,
+        lastUpdate: new Date(Date.now() - 120_000),
+      },
+      consecutiveMissedPolls: 2,
+    })
+    const deferred = createDeferredPromise()
+    mockClient.session.get.mockRejectedValue(new Error("missing"))
+    mockClient.session.abort.mockImplementationOnce(() => deferred.promise)
+
+    //#when
+    const interruptPromise = checkAndInterruptStaleTasks({
+      tasks: [task],
+      client: mockClient as never,
+      config: { staleTimeoutMs: 180_000, sessionGoneTimeoutMs: 60_000 },
+      concurrencyManager: mockConcurrencyManager as never,
+      notifyParentSession: mockNotify,
+      sessionStatuses: {},
+    })
+    let settled = false
+    void interruptPromise.then(() => {
+      settled = true
+    })
+
+    await Promise.resolve()
+
+    //#then
+    expect(settled).toBe(false)
+
+    deferred.resolve()
+    await interruptPromise
+
+    expect(settled).toBe(true)
+  })
+
   it("should use session-gone timeout when session is missing from status map (no progress)", async () => {
     //#given — task started 2min ago, no progress, session completely gone
     const task = createRunningTask({