fix(go): redact internal handler errors (#15746)

### What problem does this PR solve?

Refs #15743

Some Go API handlers return raw `err.Error()` strings in
`CodeServerError` responses. Those errors can include internal backend
details such as database, storage, search engine, or host information.

This PR adds a small shared `jsonInternalError` helper for handler-level
internal failures. The helper logs the raw error server-side with
request method/path context, then returns the existing generic
`common.CodeServerError.Message()` to API clients.

This first slice migrates the existing `jsonError(c,
common.CodeServerError, err.Error())` production call sites in agent,
dataset graph, file, and system handlers. It intentionally does not
close the full issue because direct `c.JSON` error responses in other
handlers remain for follow-up PRs.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

### Tests

- `/root/go/bin/go test ./internal/handler -count=1`

---------

Co-authored-by: Yingfeng <yingfeng.zhang@gmail.com>

internal/handler/error_response_test.go

@@ -0,0 +1,59 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package handler
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"ragflow/internal/common"
+
+	"github.com/gin-gonic/gin"
+)
+
+func TestJSONInternalErrorRedactsRawError(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	c.Request = httptest.NewRequest(http.MethodGet, "/api/v1/files?tenant_id=secret-tenant", nil)
+
+	jsonInternalError(c, errors.New("postgres password=secret host=10.0.0.1 table=tenant"))
+
+	if recorder.Code != http.StatusOK {
+		t.Fatalf("status=%d, want %d", recorder.Code, http.StatusOK)
+	}
+
+	var body map[string]interface{}
+	if err := json.Unmarshal(recorder.Body.Bytes(), &body); err != nil {
+		t.Fatalf("decode response body: %v", err)
+	}
+
+	if got := body["message"]; got != common.CodeServerError.Message() {
+		t.Fatalf("message=%v, want %q", got, common.CodeServerError.Message())
+	}
+	if got := body["code"]; got != float64(common.CodeServerError) {
+		t.Fatalf("code=%v, want %d", got, common.CodeServerError)
+	}
+	if strings.Contains(recorder.Body.String(), "password=secret") || strings.Contains(recorder.Body.String(), "10.0.0.1") {
+		t.Fatalf("response leaked raw error details: %s", recorder.Body.String())
+	}
+}