refactor: `ensurePr` improved testability (#15443)

lib/workers/repository/update/branch/schedule.spec.ts

lib/workers/repository/update/branch/status-checks.ts

...ers/repository/update/pr/__snapshots__/index.spec.ts.snap

lib/workers/repository/update/pr/body/changelogs.spec.ts

lib/workers/repository/update/pr/body/changelogs.ts

...kers/repository/update/pr/body/config-description.spec.ts

lib/workers/repository/update/pr/body/config-description.ts

lib/workers/repository/update/pr/body/footer.spec.ts

lib/workers/repository/update/pr/body/footer.ts

lib/workers/repository/update/pr/body/header.spec.ts

lib/workers/repository/update/pr/body/header.ts

lib/workers/repository/update/pr/body/index.spec.ts

lib/workers/repository/update/pr/body/index.ts

lib/workers/repository/update/pr/body/notes.spec.ts

lib/workers/repository/update/pr/index.spec.ts

lib/workers/repository/update/pr/index.ts

tsconfig.strict.json