fix(agent): let stopped threads start a new task and scope thread state to the latest task

Replace the stored cancel func with a handle so a stale deferred cleanup
can't delete a newer task's cancellation, and clear a thread's cancelled
flag when it submits fresh work instead of refusing to restart. Thread
progress/finished state now derives from the latest user task in the
thread rather than the whole history. Adds coverage for continuing a
stopped thread and for sanitized generation-failure messages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/application/design_service.go

@@ -72,6 +72,10 @@ type DesignService struct {
 	now                    func() time.Time
 }
 
+type agentTaskCancelHandle struct {
+	cancel context.CancelFunc
+}
+
 type GenerateResult struct {
 	Project        design.Project
 	Message        design.Message
@@ -537,14 +541,7 @@ func (s *DesignService) AgentChat(ctx context.Context, projectID string, req des
 	if err != nil {
 		return design.AgentThread{}, err
 	}
-	if threadIsCancelled(project.Messages, threadID) || s.isAgentTaskCancelled(project.ID, threadID) {
-		return design.AgentThread{
-			ThreadID:     threadID,
-			Status:       "cancelled",
-			ThreadIDType: threadIDType,
-			Project:      project,
-		}, nil
-	}
+	s.clearAgentTaskCancellation(project.ID, threadID)
 
 	now := s.now()
 	actionID := newID()
@@ -1700,7 +1697,7 @@ func (s *DesignService) isUserCancelledGeneration(ctx context.Context, projectID
 	if getErr != nil {
 		return false
 	}
-	return threadIsCancelled(project.Messages, threadID)
+	return threadHasCancelledMessage(project.Messages, threadID)
 }
 
 func detachedUserContext(ctx context.Context) context.Context {
@@ -1737,13 +1734,16 @@ func (s *DesignService) withAgentTaskCancellation(ctx context.Context, projectID
 		return ctx, func() {}
 	}
 	taskCtx, cancel := context.WithCancel(ctx)
-	s.agentTaskCancels.Store(key, context.CancelFunc(cancel))
+	handle := &agentTaskCancelHandle{cancel: cancel}
+	s.agentTaskCancels.Store(key, handle)
 	if s.isAgentTaskCancelled(projectID, threadID) {
 		cancel()
 	}
 	return taskCtx, func() {
 		cancel()
-		s.agentTaskCancels.Delete(key)
+		if current, ok := s.agentTaskCancels.Load(key); ok && current == handle {
+			s.agentTaskCancels.Delete(key)
+		}
 	}
 }
 
@@ -1754,7 +1754,9 @@ func (s *DesignService) cancelAgentTask(projectID string, threadID string) {
 	}
 	s.cancelledAgentTasks.Store(key, s.now())
 	if cancel, ok := s.agentTaskCancels.Load(key); ok {
-		if cancelFunc, ok := cancel.(context.CancelFunc); ok {
+		if handle, ok := cancel.(*agentTaskCancelHandle); ok && handle != nil {
+			handle.cancel()
+		} else if cancelFunc, ok := cancel.(context.CancelFunc); ok {
 			cancelFunc()
 		}
 	}
@@ -1763,6 +1765,14 @@ func (s *DesignService) cancelAgentTask(projectID string, threadID string) {
 	})
 }
 
+func (s *DesignService) clearAgentTaskCancellation(projectID string, threadID string) {
+	key := agentTaskKey(projectID, threadID)
+	if key == "" {
+		return
+	}
+	s.cancelledAgentTasks.Delete(key)
+}
+
 func (s *DesignService) isAgentTaskCancelled(projectID string, threadID string) bool {
 	key := agentTaskKey(projectID, threadID)
 	if key == "" {
@@ -3122,6 +3132,30 @@ func (s *DesignService) applyIncrementalGeneratedNode(ctx context.Context, proje
 }
 
 func threadIsCancelled(messages []design.Message, threadID string) bool {
+	threadID = strings.TrimSpace(threadID)
+	if threadID == "" {
+		return false
+	}
+	for index := len(messages) - 1; index >= 0; index-- {
+		message := messages[index]
+		if strings.TrimSpace(message.ThreadID) != threadID {
+			continue
+		}
+		if strings.EqualFold(strings.TrimSpace(message.Status), "cancelled") {
+			return true
+		}
+		if strings.EqualFold(strings.TrimSpace(message.Role), "user") {
+			return false
+		}
+		switch strings.ToLower(strings.TrimSpace(message.Status)) {
+		case "running", "streaming", "submitted", "success", "failed":
+			return false
+		}
+	}
+	return false
+}
+
+func threadHasCancelledMessage(messages []design.Message, threadID string) bool {
 	threadID = strings.TrimSpace(threadID)
 	if threadID == "" {
 		return false