fix: add nil guard to DocumentDAO.GetByIDs and add tests (#15649)

## Summary

`DocumentDAO.GetByIDs()` generated `WHERE id IN ()` for empty/nil ID
slices, which is invalid SQL and would fail on most databases. This PR
adds a nil guard and comprehensive tests.

### Changes

- **Modified**: `internal/dao/document.go` — Added `len(ids) == 0` guard
to `GetByIDs`
- **New**: `internal/dao/document_test.go` — 4 test cases covering
success, empty IDs, nil IDs, and no-match

### Testing

```
=== RUN   TestDocumentGetByIDs_Success   --- PASS
=== RUN   TestDocumentGetByIDs_EmptyIDs  --- PASS
=== RUN   TestDocumentGetByIDs_NilIDs    --- PASS
=== RUN   TestDocumentGetByIDs_NoMatch   --- PASS
```

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

internal/dao/document.go

@@ -135,6 +135,9 @@ func (dao *DocumentDAO) GetAllDocIDsByKBIDs(kbIDs []string) ([]map[string]string
 
 // GetByIDs retrieves documents by multiple IDs
 func (dao *DocumentDAO) GetByIDs(ids []string) ([]*entity.Document, error) {
+	if len(ids) == 0 {
+		return nil, nil
+	}
 	var documents []*entity.Document
 	err := DB.Where("id IN ?", ids).Find(&documents).Error
 	if err != nil {

internal/dao/document_test.go

@@ -0,0 +1,121 @@
+//
+//  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 dao
+
+import (
+	"testing"
+
+	"github.com/glebarez/sqlite"
+	"gorm.io/gorm"
+
+	"ragflow/internal/entity"
+)
+
+func setupDocumentTestDB(t *testing.T) *gorm.DB {
+	t.Helper()
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{
+		TranslateError: true,
+	})
+	if err != nil {
+		t.Fatalf("failed to open sqlite: %v", err)
+	}
+	if err := db.AutoMigrate(
+		&entity.Document{},
+	); err != nil {
+		t.Fatalf("failed to migrate: %v", err)
+	}
+	return db
+}
+
+func pushDocDB(t *testing.T, testDB *gorm.DB) {
+	t.Helper()
+	orig := DB
+	DB = testDB
+	t.Cleanup(func() { DB = orig })
+}
+
+func TestDocumentGetByIDs_Success(t *testing.T) {
+	db := setupDocumentTestDB(t)
+	pushDocDB(t, db)
+
+	db.Create(&entity.Document{ID: "doc1", KbID: "kb1", Name: sp("Doc 1"), CreatedBy: "user1", ParserConfig: entity.JSONMap{}})
+	db.Create(&entity.Document{ID: "doc2", KbID: "kb1", Name: sp("Doc 2"), CreatedBy: "user1", ParserConfig: entity.JSONMap{}})
+	db.Create(&entity.Document{ID: "doc3", KbID: "kb2", Name: sp("Doc 3"), CreatedBy: "user2", ParserConfig: entity.JSONMap{}})
+
+	dao := NewDocumentDAO()
+	docs, err := dao.GetByIDs([]string{"doc1", "doc3"})
+	if err != nil {
+		t.Fatalf("GetByIDs failed: %v", err)
+	}
+	if len(docs) != 2 {
+		t.Fatalf("expected 2 docs, got %d", len(docs))
+	}
+
+	ids := make(map[string]bool)
+	for _, d := range docs {
+		ids[d.ID] = true
+	}
+	if !ids["doc1"] || !ids["doc3"] {
+		t.Errorf("expected doc1 and doc3, got %v", ids)
+	}
+}
+
+func TestDocumentGetByIDs_EmptyIDs(t *testing.T) {
+	db := setupDocumentTestDB(t)
+	pushDocDB(t, db)
+
+	dao := NewDocumentDAO()
+	docs, err := dao.GetByIDs([]string{})
+	if err != nil {
+		t.Fatalf("GetByIDs failed: %v", err)
+	}
+	if docs != nil {
+		t.Errorf("expected nil for empty IDs, got %v", docs)
+	}
+}
+
+func TestDocumentGetByIDs_NilIDs(t *testing.T) {
+	db := setupDocumentTestDB(t)
+	pushDocDB(t, db)
+
+	dao := NewDocumentDAO()
+	docs, err := dao.GetByIDs(nil)
+	if err != nil {
+		t.Fatalf("GetByIDs failed: %v", err)
+	}
+	if docs != nil {
+		t.Errorf("expected nil for nil IDs, got %v", docs)
+	}
+}
+
+func TestDocumentGetByIDs_NoMatch(t *testing.T) {
+	db := setupDocumentTestDB(t)
+	pushDocDB(t, db)
+
+	db.Create(&entity.Document{ID: "doc1", KbID: "kb1", Name: sp("Doc 1"), CreatedBy: "user1", ParserConfig: entity.JSONMap{}})
+
+	dao := NewDocumentDAO()
+	docs, err := dao.GetByIDs([]string{"nonexistent"})
+	if err != nil {
+		t.Fatalf("GetByIDs failed: %v", err)
+	}
+	if len(docs) != 0 {
+		t.Errorf("expected 0 docs, got %d", len(docs))
+	}
+}
+
+func sp(s string) *string { return &s }