fix(#30300): force export 404.html (#36827)
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
The PR fixes #30300.
The previous integration test case only checks if `/out/404.html` exists. However, the test passes since `/out/404.html/index.html` is being exported instead.
The PR changes that by checking if a given path exists and is a file.
