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/handler/document.go

@@ -52,7 +52,7 @@ type documentServiceIface interface {
 	ListDocuments(page, pageSize int) ([]*service.DocumentResponse, int64, error)
 	ListDocumentsByDatasetID(kbID string, page, pageSize int) ([]*entity.DocumentListItem, int64, error)
 	GetDocumentsByAuthorID(authorID, page, pageSize int) ([]*service.DocumentResponse, int64, error)
-	GetThumbnail(docID string) (*service.ThumbnailResponse, error)
+	GetThumbnails(userID string, docIDs []string) (map[string]string, error)
 	GetDocumentImage(imageID string) ([]byte, error)
 	GetMetadataSummary(kbID string, docIDs []string) (map[string]interface{}, error)
 	SetDocumentMetadata(docID string, meta map[string]interface{}) error
@@ -168,40 +168,51 @@ func (h *DocumentHandler) GetDocumentByID(c *gin.Context) {
 
 // GetThumbnail Get thumbnails for documents.
 func (h *DocumentHandler) GetThumbnail(c *gin.Context) {
-	_, errorCode, errorMessage := GetUser(c)
+	user, errorCode, errorMessage := GetUser(c)
 	if errorCode != common.CodeSuccess {
 		jsonError(c, errorCode, errorMessage)
 		return
 	}
 
-	id := c.Query("doc_ids")
-	if id == "" {
-		c.JSON(http.StatusBadRequest, gin.H{
-			"error": errors.New("invalid document id"),
-		})
+	docIDs := parseThumbnailDocIDs(c)
+	if len(docIDs) == 0 {
+		jsonError(c, common.CodeArgumentError, `Lack of "Document ID"`)
 		return
 	}
 
-	result, err := h.documentService.GetThumbnail(id)
+	result, err := h.documentService.GetThumbnails(user.ID, docIDs)
 	if err != nil {
-		c.JSON(http.StatusNotFound, gin.H{
-			"error": fmt.Errorf("thumbnail not found"),
-		})
+		jsonError(c, common.CodeServerError, err.Error())
 		return
 	}
 
-	if result.Thumbnail != nil && *result.Thumbnail != "" {
-		newThumbURL := fmt.Sprintf("/api/v1/documents/images/%s-%s", result.KbID, *result.Thumbnail)
-		result.Thumbnail = &newThumbURL
-	}
-
 	c.JSON(http.StatusOK, gin.H{
 		"code":    common.CodeSuccess,
-		"data":    map[string]interface{}{result.ID: result.Thumbnail},
+		"data":    result,
 		"message": "success",
 	})
 }
 
+func parseThumbnailDocIDs(c *gin.Context) []string {
+	rawValues := c.QueryArray("doc_ids")
+	seen := make(map[string]struct{}, len(rawValues))
+	docIDs := make([]string, 0, len(rawValues))
+
+	for _, raw := range rawValues {
+		id := strings.TrimSpace(raw)
+		if id == "" {
+			continue
+		}
+		if _, ok := seen[id]; ok {
+			continue
+		}
+		seen[id] = struct{}{}
+		docIDs = append(docIDs, id)
+	}
+
+	return docIDs
+}
+
 // GetDocumentImage returns a document image from object storage.
 func (h *DocumentHandler) GetDocumentImage(c *gin.Context) {
 	imageID := c.Param("image_id")

internal/handler/document_test.go

@@ -42,6 +42,10 @@ type fakeDocumentService struct {
 	err             error
 	stopResult      map[string]interface{}
 	stopErr         error
+	thumbnails      map[string]string
+	thumbnailErr    error
+	thumbnailUserID string
+	thumbnailDocIDs []string
 	metadataSummary map[string]interface{}
 	metadataErr     error
 	metadataKBID    string
@@ -145,8 +149,10 @@ func (f *fakeDocumentService) ListDocumentsByDatasetID(kbID string, page, pageSi
 func (f *fakeDocumentService) BatchUpdateDocumentStatus(userID, datasetID, status string, documentIDs []string) (map[string]interface{}, common.ErrorCode, error) {
 	return map[string]interface{}{}, common.CodeSuccess, nil
 }
-func (f *fakeDocumentService) GetThumbnail(docID string) (*service.ThumbnailResponse, error) {
-	return nil, nil
+func (f *fakeDocumentService) GetThumbnails(userID string, docIDs []string) (map[string]string, error) {
+	f.thumbnailUserID = userID
+	f.thumbnailDocIDs = append([]string(nil), docIDs...)
+	return f.thumbnails, f.thumbnailErr
 }
 func (f *fakeDocumentService) GetDocumentImage(imageID string) ([]byte, error) {
 	return nil, nil
@@ -908,6 +914,48 @@ func TestMetadataSummaryByDataset_Success(t *testing.T) {
 	}
 }
 
+func TestGetThumbnail_Success(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	fake := &fakeDocumentService{
+		thumbnails: map[string]string{
+			"doc-1": "/api/v1/documents/images/kb-1-thumb-1.png",
+			"doc-2": "",
+		},
+	}
+	h := &DocumentHandler{
+		documentService: fake,
+	}
+
+	c, w := setupGinContextWithUser("GET", "/api/v1/thumbnails?doc_ids=doc-1&doc_ids=doc-2", "")
+
+	h.GetThumbnail(c)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", w.Code, w.Body.String())
+	}
+	if len(fake.thumbnailDocIDs) != 2 || fake.thumbnailDocIDs[0] != "doc-1" || fake.thumbnailDocIDs[1] != "doc-2" {
+		t.Fatalf("unexpected docIDs: %#v", fake.thumbnailDocIDs)
+	}
+	if fake.thumbnailUserID != "user-1" {
+		t.Fatalf("unexpected userID: %s", fake.thumbnailUserID)
+	}
+
+	var resp map[string]interface{}
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("failed to unmarshal response: %v", err)
+	}
+	if resp["code"] != float64(common.CodeSuccess) {
+		t.Fatalf("expected code %d, got %v", common.CodeSuccess, resp["code"])
+	}
+	data := resp["data"].(map[string]interface{})
+	if data["doc-1"] != "/api/v1/documents/images/kb-1-thumb-1.png" {
+		t.Fatalf("unexpected thumbnail for doc-1: %v", data["doc-1"])
+	}
+	if data["doc-2"] != "" {
+		t.Fatalf("unexpected thumbnail for doc-2: %v", data["doc-2"])
+	}
+}
+
 func TestGetDocumentArtifact_Success(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	h := &DocumentHandler{