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>

services/pull/pull.go

@@ -479,7 +479,7 @@ func AddTestPullRequestTask(opts TestPullRequestOptions) {
 						}
 					}
 
-					notify_service.PullRequestSynchronized(ctx, opts.Doer, pr)
+					notify_service.PullRequestSynchronized(ctx, opts.Doer, pr, opts.OldCommitID, opts.NewCommitID)
 				}
 			}
 		}