Rework suggestion backend (#33538)

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: wxiaoguang <wxiaoguang@gmail.com>

services/issue/suggestion.go

@@ -0,0 +1,73 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package issue
+
+import (
+	"context"
+	"strconv"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/modules/optional"
+	"code.gitea.io/gitea/modules/structs"
+)
+
+func GetSuggestion(ctx context.Context, repo *repo_model.Repository, isPull optional.Option[bool], keyword string) ([]*structs.Issue, error) {
+	var issues issues_model.IssueList
+	var err error
+	pageSize := 5
+	if keyword == "" {
+		issues, err = issues_model.FindLatestUpdatedIssues(ctx, repo.ID, isPull, pageSize)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		indexKeyword, _ := strconv.ParseInt(keyword, 10, 64)
+		var issueByIndex *issues_model.Issue
+		var excludedID int64
+		if indexKeyword > 0 {
+			issueByIndex, err = issues_model.GetIssueByIndex(ctx, repo.ID, indexKeyword)
+			if err != nil && !issues_model.IsErrIssueNotExist(err) {
+				return nil, err
+			}
+			if issueByIndex != nil {
+				excludedID = issueByIndex.ID
+				pageSize--
+			}
+		}
+
+		issues, err = issues_model.FindIssuesSuggestionByKeyword(ctx, repo.ID, keyword, isPull, excludedID, pageSize)
+		if err != nil {
+			return nil, err
+		}
+
+		if issueByIndex != nil {
+			issues = append([]*issues_model.Issue{issueByIndex}, issues...)
+		}
+	}
+
+	if err := issues.LoadPullRequests(ctx); err != nil {
+		return nil, err
+	}
+
+	suggestions := make([]*structs.Issue, 0, len(issues))
+	for _, issue := range issues {
+		suggestion := &structs.Issue{
+			ID:    issue.ID,
+			Index: issue.Index,
+			Title: issue.Title,
+			State: issue.State(),
+		}
+
+		if issue.IsPull && issue.PullRequest != nil {
+			suggestion.PullRequest = &structs.PullRequestMeta{
+				HasMerged:        issue.PullRequest.HasMerged,
+				IsWorkInProgress: issue.PullRequest.IsWorkInProgress(ctx),
+			}
+		}
+		suggestions = append(suggestions, suggestion)
+	}
+
+	return suggestions, nil
+}