fix: prevent background agent race condition in session prompt (#2932)

Added await for session ready state before sending prompt in
background-agent/manager.ts. Also improved image resizer error handling.

132 tests pass, tsc clean.

Closes #2932

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

@@ -1431,6 +1431,46 @@ describe("BackgroundManager.tryCompleteTask", () => {
     expect(task.concurrencyKey).toBeUndefined()
   })
 
+  test("should mark task as error when startTask throws after session creation", async () => {
+    //#given - startTask creates session but fails before sending prompt
+    const concurrencyKey = "anthropic/claude-opus-4-6"
+
+    const task = createMockTask({
+      id: "task-zombie-session",
+      parentSessionID: "parent-zombie",
+      status: "pending",
+      agent: "explore",
+    })
+    delete (task as Partial<BackgroundTask>).sessionID
+
+    const input = {
+      description: task.description,
+      prompt: task.prompt,
+      agent: task.agent,
+      parentSessionID: task.parentSessionID,
+      parentMessageID: task.parentMessageID,
+      model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+    }
+    getTaskMap(manager).set(task.id, task)
+    getQueuesByKey(manager).set(concurrencyKey, [{ task, input }])
+
+    ;(manager as unknown as { startTask: (item: { task: BackgroundTask; input: typeof input }) => Promise<void> }).startTask = async (item) => {
+      item.task.status = "running"
+      item.task.sessionID = "ses_zombie_child"
+      item.task.startedAt = new Date()
+      item.task.concurrencyKey = concurrencyKey
+      throw new Error("crash between session creation and prompt send")
+    }
+
+    //#when
+    await processKeyForTest(manager, concurrencyKey)
+
+    //#then - task must be marked as error, not left in running zombie state
+    expect(task.status).toBe("error")
+    expect(task.error).toContain("crash between session creation and prompt send")
+    expect(task.completedAt).toBeDefined()
+  })
+
   test("should release queue slot when queued task is already interrupt", async () => {
     // given
     const concurrencyKey = "anthropic/claude-opus-4-6"

src/features/background-agent/manager.ts

@@ -371,7 +371,7 @@ export class BackgroundManager {
       this.markPreStartDescendantReservation(task)
 
       // Trigger processing (fire-and-forget)
-      this.processKey(key)
+      void this.processKey(key)
 
       return { ...task }
     } catch (error) {
@@ -408,12 +408,35 @@ export class BackgroundManager {
         } catch (error) {
           log("[background-agent] Error starting task:", error)
           this.rollbackPreStartDescendantReservation(item.task)
+
+          // Mark task as error so the parent polling loop detects the failure
+          // instead of leaving it in a zombie "running" state with no prompt sent
+          item.task.status = "error"
+          item.task.error = error instanceof Error ? error.message : String(error)
+          item.task.completedAt = new Date()
+
           if (item.task.concurrencyKey) {
             this.concurrencyManager.release(item.task.concurrencyKey)
             item.task.concurrencyKey = undefined
           } else {
             this.concurrencyManager.release(key)
           }
+
+          if (item.task.rootSessionID) {
+            this.unregisterRootDescendant(item.task.rootSessionID)
+          }
+
+          removeTaskToastTracking(item.task.id)
+
+          // Abort the orphaned session if one was created before the error
+          if (item.task.sessionID) {
+            await this.abortSessionWithLogging(item.task.sessionID, "startTask error cleanup")
+          }
+
+          this.markForNotification(item.task)
+          this.enqueueNotificationForParent(item.task.parentSessionID, () => this.notifyParentSession(item.task)).catch(err => {
+            log("[background-agent] Failed to notify on startTask error:", err)
+          })
         }
       }
     } finally {