feat(concurrency): prevent background task races and leaks

Ensure queue waiters settle once, centralize completion with status guards, and release slots before async work so shutdown and cancellations don’t leak concurrency. Internal hardening only.

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

@@ -349,3 +349,70 @@ describe("ConcurrencyManager.acquire/release", () => {
     await waitPromise
   })
 })
+
+describe("ConcurrencyManager.cleanup", () => {
+  test("cancelWaiters should reject all pending acquires", async () => {
+    // #given
+    const config: BackgroundTaskConfig = { defaultConcurrency: 1 }
+    const manager = new ConcurrencyManager(config)
+    await manager.acquire("model-a")
+
+    // Queue waiters
+    const errors: Error[] = []
+    const p1 = manager.acquire("model-a").catch(e => errors.push(e))
+    const p2 = manager.acquire("model-a").catch(e => errors.push(e))
+
+    // #when
+    manager.cancelWaiters("model-a")
+    await Promise.all([p1, p2])
+
+    // #then
+    expect(errors.length).toBe(2)
+    expect(errors[0].message).toContain("cancelled")
+  })
+
+  test("clear should cancel all models and reset state", async () => {
+    // #given
+    const config: BackgroundTaskConfig = { defaultConcurrency: 1 }
+    const manager = new ConcurrencyManager(config)
+    await manager.acquire("model-a")
+    await manager.acquire("model-b")
+
+    const errors: Error[] = []
+    const p1 = manager.acquire("model-a").catch(e => errors.push(e))
+    const p2 = manager.acquire("model-b").catch(e => errors.push(e))
+
+    // #when
+    manager.clear()
+    await Promise.all([p1, p2])
+
+    // #then
+    expect(errors.length).toBe(2)
+    expect(manager.getCount("model-a")).toBe(0)
+    expect(manager.getCount("model-b")).toBe(0)
+  })
+
+  test("getCount and getQueueLength should return correct values", async () => {
+    // #given
+    const config: BackgroundTaskConfig = { defaultConcurrency: 2 }
+    const manager = new ConcurrencyManager(config)
+
+    // #when
+    await manager.acquire("model-a")
+    expect(manager.getCount("model-a")).toBe(1)
+    expect(manager.getQueueLength("model-a")).toBe(0)
+
+    await manager.acquire("model-a")
+    expect(manager.getCount("model-a")).toBe(2)
+
+    // Queue one more
+    const p = manager.acquire("model-a").catch(() => {})
+    await Promise.resolve() // let it queue
+
+    expect(manager.getQueueLength("model-a")).toBe(1)
+
+    // Cleanup
+    manager.cancelWaiters("model-a")
+    await p
+  })
+})