test(gateway): 修既有 flake — 根因在测试替身的取消契约，非产品 bug

TestInterruptCancelsTurnAndClearsPending 在纯 HEAD 与新代码上都约 7–15/50 失败（既有 flake，污染每次验收）。
- **根因（测试侧）**：测试替身 gatedRunner 阻塞在 select{tickets | ctx.Done()}；A 轮被 interrupt 取消时它已 park，ticket 释放后**同一调度步内两个 case 同时就绪**，Go 的 select 随机选 → 有时把**已被取消**的 turn 记成执行过（诊断重放 300 次：18 次失败，且每次被记执行的 ctx 都已 canceled）
- **产品侧复核无 bug**：interruptLocked 在锁内 cancel + 清 pending + gen++；被取消/顶替的轮 completeTurn 命中 stale gen 直接返回，不碰状态、不续排 pending
- **修法（仅测试）**：替身忠于真 runner 的取消契约（入口先查 ctx.Err()、两条取消路径都 recordCanceled、新增 waitCanceled）；测试在 Interrupt 后、释放 ticket 前**先等被取消轮确实停下**，断言不再依赖调度巧合；另加 waitIdle / run.count()==1 / session_idle 事件数==1（原有 4 处断言一字未删）
- **稳定性**：修前 -count=50 四次 15/9/7/10 失败 → 修后 -count=50×8 + -count=100 + -count=300 **0/750**；-race 目标测试与整包均 ok
- **变异验证**（防改断言变绿）：去掉 interrupt 的 cancel → 10/10 FAIL；不清 pending → FAIL；去掉 completeTurn 的 stale-gen 判断 → 10/10 FAIL

internal/services/gateway/session_orchestrator_test.go

@@ -26,11 +26,20 @@ type callRecord struct {
 // proceeds it records the call and returns a deterministic reply. A nil err
 // produces a reply holding the *incoming* messages as text, so a test can assert
 // exactly what context the resumed turn carried.
+//
+// Like the real loop runner, it never executes a turn whose context is already
+// canceled, and it counts those canceled returns so a test can wait for an
+// interrupted turn to actually stop (waitCanceled) before releasing the next
+// ticket. Without that, a turn that was canceled *while parked in the select
+// below* can still win the ticket case in the same scheduler step (both cases are
+// ready) and be recorded as if it had run — a test-harness artifact, not an
+// orchestrator ordering guarantee.
 type gatedRunner struct {
-	mu      sync.Mutex
-	calls   []callRecord
-	tickets chan struct{}
-	err     error
+	mu       sync.Mutex
+	calls    []callRecord
+	canceled int
+	tickets  chan struct{}
+	err      error
 }
 
 func newGatedRunner(cap int) *gatedRunner {
@@ -38,9 +47,14 @@ func newGatedRunner(cap int) *gatedRunner {
 }
 
 func (g *gatedRunner) Run(ctx context.Context, sessionID string, messages []model.Message, env loop.SessionEnv) (*loop.Result, error) {
+	if err := ctx.Err(); err != nil {
+		g.recordCanceled()
+		return nil, err
+	}
 	select {
 	case <-g.tickets:
 	case <-ctx.Done():
+		g.recordCanceled()
 		return nil, ctx.Err()
 	}
 	g.mu.Lock()
@@ -78,6 +92,47 @@ func (g *gatedRunner) count() int {
 	return len(g.calls)
 }
 
+// recordCanceled notes one invocation that returned because its context was
+// canceled, i.e. a turn that never executed.
+func (g *gatedRunner) recordCanceled() {
+	g.mu.Lock()
+	g.canceled++
+	g.mu.Unlock()
+}
+
+// waitCanceled blocks until at least n invocations have stopped because their
+// context was canceled, reporting false when that never happens within the
+// deadline. It is the barrier a test uses between an Interrupt and the release of
+// the next ticket: once it holds, no canceled turn can still consume a ticket.
+func (g *gatedRunner) waitCanceled(n int) bool {
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		g.mu.Lock()
+		ok := g.canceled >= n
+		g.mu.Unlock()
+		if ok {
+			return true
+		}
+		time.Sleep(time.Millisecond)
+	}
+	return false
+}
+
+// waitIdle blocks until the session reports idle with an empty pending queue,
+// reporting false when it never settles within the deadline. It lets a test assert
+// the settled state deterministically instead of sleeping, and turns "a dropped
+// turn was resumed anyway" into a test failure rather than a silent pass.
+func waitIdle(o *Orchestrator, sessionID string) bool {
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		if !o.Busy(sessionID) && o.QueueDepth(sessionID) == 0 {
+			return true
+		}
+		time.Sleep(time.Millisecond)
+	}
+	return false
+}
+
 // eventRec is one state transition captured by the recording sink.
 type eventRec struct {
 	sessionID, typ string
@@ -556,7 +611,7 @@ func TestSubmitPendingBoundedRejectsOverflow(t *testing.T) {
 func TestInterruptCancelsTurnAndClearsPending(t *testing.T) {
 	t.Parallel()
 	run := newGatedRunner(2)
-	o, _ := seedOrchestrator(run, 5)
+	o, sink := seedOrchestrator(run, 5)
 
 	if sr := o.Submit(context.Background(), "s1", []model.Message{userMsg("A")}, loop.OwnerEnv()); !sr.Immediate {
 		t.Fatalf("A = %+v, want Immediate", sr)
@@ -574,6 +629,14 @@ func TestInterruptCancelsTurnAndClearsPending(t *testing.T) {
 		t.Fatalf("session not idle after interrupt: busy=%v depth=%d", o.Busy("s1"), o.QueueDepth("s1"))
 	}
 
+	// The cancel must reach the in-flight turn (design/15 §3: interrupt is a
+	// graceful context cancel) and it must stop before any further turn is
+	// released: a canceled turn never executes, so it can never consume the ticket
+	// meant for the next one.
+	if !run.waitCanceled(1) {
+		t.Fatal("interrupted turn never observed its context cancellation")
+	}
+
 	// A new message after the interrupt must start fresh (Immediate), and the
 	// canceled chain must not have resumed B.
 	if sr := o.Submit(context.Background(), "s1", []model.Message{userMsg("C")}, loop.OwnerEnv()); !sr.Immediate {
@@ -587,6 +650,28 @@ func TestInterruptCancelsTurnAndClearsPending(t *testing.T) {
 	if messageText(got[0].messages[0]) != "C" {
 		t.Fatalf("only surviving call = %v, want the C turn", messageTextList(got[0].messages))
 	}
+
+	// C's chain settles with nothing left to run: had the interrupt failed to
+	// cancel A or to clear B, the surviving chain would have resumed a turn and the
+	// session would never go idle again.
+	if !waitIdle(o, "s1") {
+		t.Fatalf("session never settled idle: busy=%v depth=%d", o.Busy("s1"), o.QueueDepth("s1"))
+	}
+	if n := run.count(); n != 1 {
+		t.Fatalf("runner calls = %d, want 1 (neither the canceled A nor the dropped B may execute)", n)
+	}
+	// The canceled turn's completion is a no-op for session state (its generation is
+	// stale), so it must not emit the idle event of the turn now running: exactly
+	// one session_idle belongs to this scenario (C settling).
+	idles := 0
+	for _, ty := range sink.types() {
+		if ty == EventSessionIdle {
+			idles++
+		}
+	}
+	if idles != 1 {
+		t.Fatalf("session_idle events = %d, want 1 (a canceled turn must not settle the fresh chain)", idles)
+	}
 }
 
 func TestSubmitConcurrentSameSessionIsSafeAndBounded(t *testing.T) {