Rework suggestion backend (#33538) (#33546)

Backport #33538 by @lunny

Fix #33522 

The suggestion backend logic now is

- If the keyword is empty, returned the latest 5 issues/prs with index
desc order
- If the keyword is digital, find all issues/prs which `index` has a
prefix with that, with index asc order
- If the keyword is non-digital or if the queried records less than 5,
searching issues/prs title with a `like`, with index desc order

## Empty keyword
<img width="310" alt="image"
src="https://github.com/user-attachments/assets/1912c634-0d98-4eeb-8542-d54240901f77"
/>

## Digital
<img width="479" alt="image"
src="https://github.com/user-attachments/assets/0356a936-7110-4a24-b21e-7400201bf9b8"
/>

## Digital and title contains the digital
<img width="363" alt="image"
src="https://github.com/user-attachments/assets/6e12f908-28fe-48de-8ccc-09cbeab024d4"
/>

## non-Digital
<img width="435" alt="image"
src="https://github.com/user-attachments/assets/2722bb53-baa2-4d67-a224-522a65f73856"
/>
<img width="477" alt="image"
src="https://github.com/user-attachments/assets/06708dd9-80d1-4a88-b32b-d29072dd1ba6"
/>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/issue/suggestion_test.go

@@ -0,0 +1,57 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package issue
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/optional"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_Suggestion(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+
+	testCases := []struct {
+		keyword         string
+		isPull          optional.Option[bool]
+		expectedIndexes []int64
+	}{
+		{
+			keyword:         "",
+			expectedIndexes: []int64{5, 1, 4, 2, 3},
+		},
+		{
+			keyword:         "1",
+			expectedIndexes: []int64{1},
+		},
+		{
+			keyword:         "issue",
+			expectedIndexes: []int64{4, 1, 2, 3},
+		},
+		{
+			keyword:         "pull",
+			expectedIndexes: []int64{5},
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.keyword, func(t *testing.T) {
+			issues, err := GetSuggestion(db.DefaultContext, repo1, testCase.isPull, testCase.keyword)
+			assert.NoError(t, err)
+
+			issueIndexes := make([]int64, 0, len(issues))
+			for _, issue := range issues {
+				issueIndexes = append(issueIndexes, issue.Index)
+			}
+			assert.EqualValues(t, testCase.expectedIndexes, issueIndexes)
+		})
+	}
+}