fix(background-agent): handle session.error and prevent zombie queue starts

Marks background tasks as error on session.error to release concurrency immediately, and skips/removes error tasks from queues to avoid zombie starts.

src/features/background-agent/task-queue-processor.ts

@@ -27,7 +27,7 @@ export async function processConcurrencyKeyQueue(args: {
 
       await concurrencyManager.acquire(key)
 
-      if (item.task.status === "cancelled") {
+      if (item.task.status === "cancelled" || item.task.status === "error") {
         concurrencyManager.release(key)
         queue.shift()
         continue