fix: return call failed when LLM not available (#16518)

### Summary

As title

internal/service/ask_service.go

@@ -18,6 +18,7 @@ package service
 
 import (
 	"context"
+	"strings"
 
 	"ragflow/internal/common"
 	modelModule "ragflow/internal/entity/models"
@@ -208,6 +209,11 @@ func (s *AskService) run(ctx context.Context, llm StreamingLLM, userID, question
 
 	// Phase 4: Finalize — citation insertion + reference formatting.
 	visible := ExtractVisibleAnswer(fullAnswer)
+	if strings.TrimSpace(visible) == "" {
+		common.Warn("AskService LLM stream completed without visible answer")
+		s.sendOrCancel(out, AskDelta{Kind: AskDeltaError, Value: "LLM call failed"}, ctx)
+		return
+	}
 	chunkRefs := ChunksFormat(chunks)
 
 	// Attempt citation insertion if embedder is available.

internal/service/ask_service_test.go

@@ -114,6 +114,21 @@ func TestAskService_StreamingFlow(t *testing.T) {
 	}
 }
 
+func TestAskService_EmptyLLMStreamReturnsError(t *testing.T) {
+	ret := &fakeRetriever{result: &RetrievalTestResponse{
+		Chunks: []map[string]interface{}{
+			{"id": "c1", "content_with_weight": "test chunk", "docnm_kwd": "Doc", "kb_id": "kb1", "doc_id": "d1"},
+		},
+	}}
+	llm := &fakeStreamLLM{}
+	svc := NewAskService(ret, nil, 0, 0)
+	deltas := collect(svc.Stream(context.Background(), llm, "user1", "test", []string{"kb1"}))
+
+	if len(deltas) != 1 || deltas[0].Kind != AskDeltaError || !strings.Contains(deltas[0].Value, "LLM call failed") {
+		t.Fatalf("expected LLM error delta, got %+v", deltas)
+	}
+}
+
 func TestAskService_ThinkTags(t *testing.T) {
 	ret := &fakeRetriever{result: &RetrievalTestResponse{
 		Chunks: []map[string]interface{}{