feat(actions): add before/after to PR synchronize event payload (#37827)

## Summary

- Add `before` and `after` fields to `PullRequestPayload` for
`synchronize` events
- Thread push old/new commit SHAs through the PR synchronize notifier
path (regular and Agit flows)
- Populate the fields in webhook and Actions event payloads so workflows
can access them via `github.event.before` and `github.event.after`

Fixes #33395

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

modules/structs/hook_test.go

@@ -0,0 +1,59 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package structs
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/json"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPullRequestPayloadSynchronizeBeforeAfter(t *testing.T) {
+	payload := &PullRequestPayload{
+		Action: HookIssueSynchronized,
+		Before: "1111111111111111111111111111111111111111",
+		After:  "2222222222222222222222222222222222222222",
+		Index:  12,
+	}
+
+	data, err := json.Marshal(payload)
+	require.NoError(t, err)
+
+	assert.JSONEq(t, `{
+		"action": "synchronized",
+		"before": "1111111111111111111111111111111111111111",
+		"after": "2222222222222222222222222222222222222222",
+		"number": 12,
+		"commit_id": "",
+		"pull_request": null,
+		"repository": null,
+		"requested_reviewer": null,
+		"review": null,
+		"sender": null
+	}`, string(data))
+}
+
+func TestPullRequestPayloadNonSynchronizeOmitsBeforeAfter(t *testing.T) {
+	payload := &PullRequestPayload{
+		Action: HookIssueOpened,
+		Index:  12,
+	}
+
+	data, err := json.Marshal(payload)
+	require.NoError(t, err)
+
+	assert.JSONEq(t, `{
+		"action": "opened",
+		"number": 12,
+		"commit_id": "",
+		"pull_request": null,
+		"repository": null,
+		"requested_reviewer": null,
+		"review": null,
+		"sender": null
+	}`, string(data))
+}