feat(go-api): migrate datasets tags aggregation API to Go (#16181)

### Description

Migrates the datasets tags aggregation API `GET
/api/v1/datasets/tags/aggregation` from Python to Go.

### Changes
- Registered the `GET /api/v1/datasets/tags/aggregation` route.
- Implemented `AggregateTags` in datasets `handler` and `service`.
- Added handler and service `unit tests`.

### Test Verification
- Verified by comparing results between Python (9380) and Go (9384)
services.
- Tested scenarios: single dataset, multiple datasets, empty parameters,
and unauthorized/invalid IDs.
- All tests and Go `unit tests` passed.

internal/handler/datasets.go

@@ -592,6 +592,44 @@ func (h *DatasetsHandler) RemoveTags(c *gin.Context) {
 	jsonResponse(c, common.CodeSuccess, true, "success")
 }
 
+// AggregateTags handles GET /api/v1/datasets/tags/aggregation.
+// @Summary Aggregate dataset tags
+// @Description Aggregate tags across multiple datasets
+// @Tags datasets
+// @Produce json
+// @Security ApiKeyAuth
+// @Param dataset_ids query string true "Comma-separated dataset IDs"
+// @Success 200 {object} map[string]interface{}
+// @Router /api/v1/datasets/tags/aggregation [get]
+func (h *DatasetsHandler) AggregateTags(c *gin.Context) {
+	user, errorCode, errorMessage := GetUser(c)
+	if errorCode != common.CodeSuccess {
+		jsonError(c, errorCode, errorMessage)
+		return
+	}
+
+	rawIDs := strings.Split(c.Query("dataset_ids"), ",")
+	datasetIDs := make([]string, 0, len(rawIDs))
+
+	for _, rawID := range rawIDs {
+		tempID := strings.TrimSpace(rawID)
+		if tempID != "" {
+			datasetIDs = append(datasetIDs, tempID)
+		}
+	}
+	if len(datasetIDs) == 0 {
+		jsonError(c, common.CodeDataError, "Lack of dataset_ids in query parameters")
+		return
+	}
+
+	result, code, err := h.datasetsService.AggregateTags(datasetIDs, user.ID)
+	if err != nil {
+		jsonError(c, code, err.Error())
+		return
+	}
+	jsonResponse(c, common.CodeSuccess, result, "success")
+}
+
 // RunIndex Run an indexing task (graph/raptor/mindmap) for a dataset.
 func (h *DatasetsHandler) RunIndex(c *gin.Context) {
 	user, errorCode, errorMessage := GetUser(c)

internal/handler/datasets_aggregate_tags_test.go

@@ -0,0 +1,78 @@
+package handler
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+
+	"ragflow/internal/common"
+	"ragflow/internal/entity"
+)
+
+func newAggregateTagsHandlerRouter(authenticated bool) *gin.Engine {
+	gin.SetMode(gin.TestMode)
+	h := &DatasetsHandler{}
+	r := gin.New()
+	r.GET("/api/v1/datasets/tags/aggregation", func(c *gin.Context) {
+		if authenticated {
+			c.Set("user", &entity.User{ID: "user-1"})
+		}
+		h.AggregateTags(c)
+	})
+	return r
+}
+
+func TestDatasetsHandlerAggregateTagsRequiresDatasetIDs(t *testing.T) {
+	r := newAggregateTagsHandlerRouter(true)
+
+	resp := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/datasets/tags/aggregation", nil)
+	r.ServeHTTP(resp, req)
+
+	if resp.Code != http.StatusOK {
+		t.Fatalf("status=%d body=%s", resp.Code, resp.Body.String())
+	}
+
+	var body struct {
+		Code    int         `json:"code"`
+		Data    interface{} `json:"data"`
+		Message string      `json:"message"`
+	}
+	if err := json.Unmarshal(resp.Body.Bytes(), &body); err != nil {
+		t.Fatalf("unmarshal response: %v body=%s", err, resp.Body.String())
+	}
+	if body.Code != int(common.CodeDataError) {
+		t.Fatalf("code=%d want=%d body=%s", body.Code, common.CodeDataError, resp.Body.String())
+	}
+	if body.Message != "Lack of dataset_ids in query parameters" {
+		t.Fatalf("message=%q want=%q", body.Message, "Lack of dataset_ids in query parameters")
+	}
+	if body.Data != nil {
+		t.Fatalf("data=%v want nil", body.Data)
+	}
+}
+
+func TestDatasetsHandlerAggregateTagsRequiresAuth(t *testing.T) {
+	r := newAggregateTagsHandlerRouter(false)
+
+	resp := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/datasets/tags/aggregation?dataset_ids=123e4567-e89b-12d3-a456-426614174000", nil)
+	r.ServeHTTP(resp, req)
+
+	var body struct {
+		Code    int    `json:"code"`
+		Message string `json:"message"`
+	}
+	if err := json.Unmarshal(resp.Body.Bytes(), &body); err != nil {
+		t.Fatalf("unmarshal response: %v body=%s", err, resp.Body.String())
+	}
+	if body.Code != int(common.CodeUnauthorized) {
+		t.Fatalf("code=%d want=%d body=%s", body.Code, common.CodeUnauthorized, resp.Body.String())
+	}
+	if body.Message != "User not found" {
+		t.Fatalf("message=%q want=%q", body.Message, "User not found")
+	}
+}