Fix the path check in a file export endpoint

Your app has an endpoint that serves a user's exported report by file name. The first version joined the name straight onto the reports directory, so ../../etc/passwd served a system file. The fix has to turn the name into a path that stays inside the directory, or reject it.

An assistant wrote resolve_export_path(reports_dir, requested_name). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• reports_dir: the reports directory as an absolute path, such as /srv/app/reports. It may end with / and has no . or .. parts.
• requested_name: the file name the user sent.
• Return the resolved absolute path, or "" to reject the request.

Rules the tests check:

• Reject an empty name and a name that starts with /.
• Split both paths on /. Empty parts and . parts are skipped.
• Walk the name's parts from inside the reports directory. A normal part goes one level down. .. goes one level up.
• Reject the request as soon as a .. would climb above the reports directory, even if later parts would come back inside it.
• Reject a name that resolves to the reports directory itself.
• Otherwise return /, then the directory's parts and the remaining name parts joined with /.