fix: PR merge (#39442)

* Revert the behavior introduced by #30805
* Now the PR status is still managed in Gitea's code where the operation
is triggerred but not in post-receive hook
* Fix #39254 and many more related bugs.
    * Fix #39124

```
// MarkAsMerged sets a pull request to merged and closes the corresponding issue
// To make sure the pull request is marked as merged correctly, the caller uses multiple-stage operations:
//  1. Create a temp repo from base, merge the head into the temp repo, and get the merged commit ID and timestamp,
//  2. The merged commit ID and related information are stored into pull request
//  3. Push the merged commit to the base repo
//  4. Call MarkAsMerged to mark the pull request as merged and do post-processing (notification, close issues, etc)
//
// If failure occurs in step 1/2/3: the pull request is still open, the base repo is not changed, the doer can start a new merge.
// If failure occurs in step 4: the pull request can be marked as merged by the merged commit ID stored in it later.
```

modelmigration/v28/v355.go

@@ -0,0 +1,28 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v28
+
+import (
+	"context"
+
+	"gitea.dev/modelmigration/base"
+
+	"xorm.io/xorm"
+)
+
+type pullAutoMerge struct {
+	MergedCommitID string `xorm:"VARCHAR(64)"`
+}
+
+func (pullAutoMerge) TableName() string {
+	return "pull_auto_merge"
+}
+
+func AddAutoMergeMergedCommitID(_ context.Context, x base.EngineMigration) error {
+	_, err := x.SyncWithOptions(xorm.SyncOptions{
+		IgnoreConstrains:  true,
+		IgnoreDropIndices: true,
+	}, new(pullAutoMerge))
+	return err
+}