test: Add e2e test for pull_request_number on push event after PR merge

Currently, there are no end-to-end (e2e) tests to verify that the pull_request_number dynamic variable is correctly populated when a push event is triggered by a pull request merge. This leaves a gap in our test coverage and could lead to undetected regressions.

This issue, inspired by the discussion in #2118, is to add an e2e test.