Fix automerge will not work because of some events haven't been triggered (#30780)

Replace #25741
Close #24445
Close #30658
Close #20646
~Depends on #30805~

Since #25741 has been rewritten totally, to make the contribution
easier, I will continue the work in this PR. Thanks @6543

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/automerge/notify.go

@@ -0,0 +1,46 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package automerge
+
+import (
+	"context"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/log"
+	notify_service "code.gitea.io/gitea/services/notify"
+)
+
+type automergeNotifier struct {
+	notify_service.NullNotifier
+}
+
+var _ notify_service.Notifier = &automergeNotifier{}
+
+// NewNotifier create a new automergeNotifier notifier
+func NewNotifier() notify_service.Notifier {
+	return &automergeNotifier{}
+}
+
+func (n *automergeNotifier) PullRequestReview(ctx context.Context, pr *issues_model.PullRequest, review *issues_model.Review, comment *issues_model.Comment, mentions []*user_model.User) {
+	// as a missing / blocking reviews could have blocked a pending automerge let's recheck
+	if review.Type == issues_model.ReviewTypeApprove {
+		if err := StartPRCheckAndAutoMergeBySHA(ctx, review.CommitID, pr.BaseRepo); err != nil {
+			log.Error("StartPullRequestAutoMergeCheckBySHA: %v", err)
+		}
+	}
+}
+
+func (n *automergeNotifier) PullReviewDismiss(ctx context.Context, doer *user_model.User, review *issues_model.Review, comment *issues_model.Comment) {
+	if err := review.LoadIssue(ctx); err != nil {
+		log.Error("LoadIssue: %v", err)
+		return
+	}
+	if err := review.Issue.LoadPullRequest(ctx); err != nil {
+		log.Error("LoadPullRequest: %v", err)
+		return
+	}
+	// as reviews could have blocked a pending automerge let's recheck
+	StartPRCheckAndAutoMerge(ctx, review.Issue.PullRequest)
+}