fix(ai): hand an unanswered external turn out again after 3 minutes

참고 측정 1세트 첫 진단에서 클라이언트가 /external/next 로 가져간 2번째 턴의 출력을
잃어 답이 오지 않았고, 턴은 awaiting_reply 에 남아 진단이 데드라인까지 멈췄다. 가져간
뒤 RequeueAfter(기본 3분) 동안 답이 없는 턴은 새 reply_token 으로 큐 앞에 되돌려
다음 폴링이 다시 받게 한다(requeued 횟수 기록, 이전 토큰은 거부). 출력 유실은
재시도 한 번의 비용으로 끝나고 진단은 이어진다.

ai/internal/external/responder.go

@@ -33,6 +33,7 @@ const maxConvCached = 64
 type Turn struct {
 	TurnID          string        `json:"turn_id"`
 	ReplyToken      string        `json:"reply_token"` // random, handed out with the turn, required by /external/reply
+	Requeued        int           `json:"requeued"`    // times this turn was handed out again after an unanswered take
 	Session         string        `json:"session"`     // first response id of the conversation ("" on the first turn)
 	Model           string        `json:"model"`
 	Turn            int           `json:"turn"` // 1-based index within the conversation
@@ -63,6 +64,7 @@ type replyItem struct {
 type pending struct {
 	turn    Turn
 	reply   chan Reply
+	takenAt time.Time // zero while queued
 }
 
 type conversation struct {
@@ -75,6 +77,7 @@ type conversation struct {
 // Responder implements agent.Responder; one instance serves every external-* diagnosis.
 type Responder struct {
 	TurnTimeout  time.Duration // how long one turn waits for the client (bounded by the caller's context as well)
+	RequeueAfter time.Duration // a taken turn with no reply after this long goes back to the queue with a new token
 
 	mu       sync.Mutex
 	queue    []*pending          // issued, not yet taken by a client
@@ -91,7 +94,7 @@ func New(turnTimeout time.Duration) *Responder {
 	if turnTimeout <= 0 {
 		turnTimeout = 20 * time.Minute
 	}
-	return &Responder{TurnTimeout: turnTimeout, taken: map[string]*pending{}, conv: map[string]conversation{}, notify: make(chan struct{}, 1)}
+	return &Responder{TurnTimeout: turnTimeout, RequeueAfter: 3 * time.Minute, taken: map[string]*pending{}, conv: map[string]conversation{}, notify: make(chan struct{}, 1)}
 }
 
 // CreateResponse queues the turn and blocks until the client replies, the context ends or TurnTimeout passes.
@@ -200,6 +203,27 @@ func argString(raw json.RawMessage) string {
 	return s
 }
 
+// requeueStale (mu held) returns taken turns whose reply never came (client lost the output, crashed, was replaced)
+// to the front of the queue with a fresh reply token, so the next poll serves them again; the old token is void.
+func (r *Responder) requeueStale() {
+	if r.RequeueAfter <= 0 {
+		return
+	}
+	var stale []*pending
+	for id, p := range r.taken {
+		if time.Since(p.takenAt) >= r.RequeueAfter {
+			delete(r.taken, id)
+			p.turn.ReplyToken = randHex(16)
+			p.turn.Requeued++
+			p.takenAt = time.Time{}
+			stale = append(stale, p)
+		}
+	}
+	if len(stale) > 0 {
+		r.queue = append(stale, r.queue...)
+	}
+}
+
 func (r *Responder) drop(id string) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
@@ -223,9 +247,11 @@ func (r *Responder) Next(w http.ResponseWriter, req *http.Request) {
 	for {
 		r.mu.Lock()
 		r.lastPoll = time.Now()
+		r.requeueStale()
 		if len(r.queue) > 0 {
 			p := r.queue[0]
 			r.queue = r.queue[1:]
+			p.takenAt = time.Now()
 			r.taken[p.turn.TurnID] = p
 			r.mu.Unlock()
 			writeJSON(w, 200, p.turn)
@@ -300,7 +326,7 @@ func (r *Responder) ReplyHandler(w http.ResponseWriter, req *http.Request) {
 func (r *Responder) Status() map[string]any {
 	r.mu.Lock()
 	defer r.mu.Unlock()
-	out := map[string]any{"queued": len(r.queue), "awaiting_reply": len(r.taken), "turn_timeout_s": int(r.TurnTimeout.Seconds())}
+	out := map[string]any{"queued": len(r.queue), "awaiting_reply": len(r.taken), "turn_timeout_s": int(r.TurnTimeout.Seconds()), "requeue_after_s": int(r.RequeueAfter.Seconds())}
 	if !r.lastPoll.IsZero() {
 		out["last_poll_age_s"] = int(time.Since(r.lastPoll).Seconds())
 	}

ai/internal/external/responder_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -137,6 +138,44 @@ func TestDeadlinesAndErrors(t *testing.T) {
 	}
 }
 
+// A taken turn whose reply never comes is handed out again after RequeueAfter with a new token; the old token is
+// refused and the new one completes the turn.
+func TestRequeueUnansweredTurn(t *testing.T) {
+	r := New(10 * time.Second)
+	r.RequeueAfter = 200 * time.Millisecond
+	srv := httptest.NewServer(r.Handler())
+	defer srv.Close()
+	ch := make(chan error, 1)
+	go func() {
+		resp, err := r.CreateResponse(context.Background(), openai.Request{Model: "external-x", Input: []openai.Item{{Type: "message", Role: "user", Content: "hi"}}})
+		if err == nil && len(resp.FunctionCalls()) != 1 {
+			err = fmt.Errorf("unexpected response %+v", resp)
+		}
+		ch <- err
+	}()
+	first := poll(t, srv.URL) // taken, never answered
+	if first.Requeued != 0 {
+		t.Fatalf("fresh turn marked requeued: %+v", first)
+	}
+	if code := get(t, srv.URL+"/external/next?wait=0"); code != 204 {
+		t.Fatalf("taken turn must not be served again at once, got %d", code)
+	}
+	time.Sleep(250 * time.Millisecond)
+	second := poll(t, srv.URL)
+	if second.TurnID != first.TurnID || second.ReplyToken == first.ReplyToken || second.Requeued != 1 {
+		t.Fatalf("requeue: first=%+v second=%+v", first, second)
+	}
+	if code, _ := post(t, srv.URL+"/external/reply", `{"turn_id":"`+first.TurnID+`","reply_token":"`+first.ReplyToken+`","output":[{"type":"function_call","name":"x","arguments":{}}]}`); code != 403 {
+		t.Fatalf("old token must be refused, got %d", code)
+	}
+	if code, _ := post(t, srv.URL+"/external/reply", `{"turn_id":"`+second.TurnID+`","reply_token":"`+second.ReplyToken+`","output":[{"type":"function_call","name":"x","arguments":{}}]}`); code != 200 {
+		t.Fatalf("new token must be accepted, got %d", code)
+	}
+	if err := <-ch; err != nil {
+		t.Fatal(err)
+	}
+}
+
 func poll(t *testing.T, base string) Turn {
 	t.Helper()
 	for i := 0; i < 20; i++ {