fix(go): agent explore thumbnail loading for multiple doc_ids (#16514)

## Summary
- align the Go `/api/v1/thumbnails` endpoint with the frontend request
format for repeated `doc_ids`
- return thumbnail mappings for multiple documents instead of failing on
a single missing document
- preserve Python-compatible thumbnail formatting, including base64
thumbnail passthrough

internal/service/document.go

@@ -141,6 +141,8 @@ type ThumbnailResponse struct {
 	KbID      string  `json:"kb_id"`
 }
 
+const imgBase64Prefix = "data:image/png;base64,"
+
 type ArtifactResponse struct {
 	Data            []byte
 	ContentType     string
@@ -874,17 +876,48 @@ func (s *DocumentService) ListDocuments(page, pageSize int) ([]*DocumentResponse
 	return responses, total, nil
 }
 
-func (s *DocumentService) GetThumbnail(docID string) (*ThumbnailResponse, error) {
-	document, err := s.documentDAO.GetByID(docID)
-	if err != nil {
-		return nil, err
+func (s *DocumentService) GetThumbnails(userID string, docIDs []string) (map[string]string, error) {
+	if len(docIDs) == 0 {
+		return map[string]string{}, nil
 	}
 
-	var result ThumbnailResponse
-	result.ID = document.ID
-	result.Thumbnail = document.Thumbnail
-	result.KbID = document.KbID
-	return &result, nil
+	tenantIDs := []string{userID}
+	if userID != "" {
+		ids, err := dao.NewUserTenantDAO().GetTenantIDsByUserID(userID)
+		if err != nil {
+			return nil, fmt.Errorf("failed to fetch user tenants: %w", err)
+		}
+		tenantIDs = append(tenantIDs, ids...)
+	}
+
+	documents, err := s.documentDAO.GetByIDsAndTenantIDs(docIDs, tenantIDs)
+	if err != nil {
+		return nil, fmt.Errorf("failed to fetch document thumbnails: %w", err)
+	}
+
+	result := make(map[string]string, len(documents))
+	for _, document := range documents {
+		if document == nil {
+			continue
+		}
+
+		thumbnail := ""
+		if document.Thumbnail != nil && *document.Thumbnail != "" {
+			if strings.HasPrefix(*document.Thumbnail, imgBase64Prefix) {
+				thumbnail = *document.Thumbnail
+			} else {
+				thumbnail = fmt.Sprintf(
+					"/api/v1/documents/images/%s-%s",
+					document.KbID,
+					*document.Thumbnail,
+				)
+			}
+		}
+
+		result[document.ID] = thumbnail
+	}
+
+	return result, nil
 }
 
 func (s *DocumentService) BatchUpdateDocumentStatus(userID, datasetID, status string, documentIDs []string) (map[string]interface{}, common.ErrorCode, error) {

internal/service/document_test.go

@@ -2230,3 +2230,88 @@ func TestGetDocumentArtifact_AuthGate(t *testing.T) {
 		t.Errorf("user-2 with unrelated session: want ErrArtifactNotFound, got %v", err)
 	}
 }
+
+func TestGetThumbnails_AlignsWithPythonFormatting(t *testing.T) {
+	db := setupServiceTestDB(t)
+	pushServiceDB(t, db)
+
+	if err := db.AutoMigrate(&entity.Document{}, &entity.Knowledgebase{}, &entity.UserTenant{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+
+	insertTestKB(t, "kb-1", "tenant-1", 0, 0, 0)
+	insertTestKB(t, "kb-2", "tenant-1", 0, 0, 0)
+	insertTestKB(t, "kb-other", "tenant-other", 0, 0, 0)
+	if err := db.Create(&entity.UserTenant{
+		ID:        "user-1_tenant-1",
+		UserID:    "user-1",
+		TenantID:  "tenant-1",
+		Role:      "owner",
+		InvitedBy: "user-1",
+		Status:    sptr("1"),
+	}).Error; err != nil {
+		t.Fatalf("seed user tenant: %v", err)
+	}
+
+	base64Thumb := "data:image/png;base64,AAAA"
+	fileThumb := "thumb.png"
+	otherThumb := "secret.png"
+	if err := db.Create(&entity.Document{
+		ID:           "doc-file",
+		KbID:         "kb-1",
+		Thumbnail:    &fileThumb,
+		ParserID:     "naive",
+		ParserConfig: entity.JSONMap{},
+		SourceType:   "local",
+		Type:         "pdf",
+		CreatedBy:    "user-1",
+		Suffix:       "png",
+	}).Error; err != nil {
+		t.Fatalf("seed file thumbnail doc: %v", err)
+	}
+	if err := db.Create(&entity.Document{
+		ID:           "doc-base64",
+		KbID:         "kb-2",
+		Thumbnail:    &base64Thumb,
+		ParserID:     "naive",
+		ParserConfig: entity.JSONMap{},
+		SourceType:   "local",
+		Type:         "pdf",
+		CreatedBy:    "user-1",
+		Suffix:       "png",
+	}).Error; err != nil {
+		t.Fatalf("seed base64 thumbnail doc: %v", err)
+	}
+	if err := db.Create(&entity.Document{
+		ID:           "doc-other",
+		KbID:         "kb-other",
+		Thumbnail:    &otherThumb,
+		ParserID:     "naive",
+		ParserConfig: entity.JSONMap{},
+		SourceType:   "local",
+		Type:         "pdf",
+		CreatedBy:    "user-other",
+		Suffix:       "png",
+	}).Error; err != nil {
+		t.Fatalf("seed other tenant thumbnail doc: %v", err)
+	}
+
+	svc := testDocumentService(t)
+	got, err := svc.GetThumbnails("user-1", []string{"doc-file", "doc-base64", "doc-other", "missing-doc"})
+	if err != nil {
+		t.Fatalf("GetThumbnails failed: %v", err)
+	}
+
+	if got["doc-file"] != "/api/v1/documents/images/kb-1-thumb.png" {
+		t.Fatalf("unexpected file thumbnail: %q", got["doc-file"])
+	}
+	if got["doc-base64"] != base64Thumb {
+		t.Fatalf("unexpected base64 thumbnail: %q", got["doc-base64"])
+	}
+	if _, ok := got["missing-doc"]; ok {
+		t.Fatalf("did not expect missing doc in result: %#v", got)
+	}
+	if _, ok := got["doc-other"]; ok {
+		t.Fatalf("did not expect other tenant doc in result: %#v", got)
+	}
+}