refactor(automerge): fix error handling, populate recent automerge tasks on restart (#39001)

* Refactor "automerge" related code, clarify many details (including "unique queue item", start check by pull head or commit)
* Fix automerge queue handler's error handling, clarify error messages
* Populate recent automerge tasks on restart to restore the previous aborted automerge tasks

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

services/pull/ref_test.go

@@ -0,0 +1,22 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package pull_test
+
+import (
+	"testing"
+
+	repo_model "gitea.dev/models/repo"
+	"gitea.dev/models/unittest"
+	"gitea.dev/services/pull"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetMergeablePullRequestsByHeadCommitID(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+	pulls, err := pull.GetMergeablePullRequestsByHeadCommitID(t.Context(), repo1, "985f0301dba5e7b34be866819cd15ad3d8f508ee")
+	assert.NoError(t, err)
+	assert.Len(t, pulls, 1)
+}