fix(files_versions): Use helper function to get versions folder

[solracsf]

Summary

Actually it throws a OCP\\Files\\NotFoundException if folder is not created/found.

Fixes:

{
  "reqId": "GN8O3iRfYZrIzBWcTFQR",
  "level": 3,
  "time": "2025-08-27T15:03:14+02:00",
  "remoteAddr": "83.13.22.4",
  "user": "user",
  "app": "no app in context",
  "method": "MOVE",
  "url": "/remote.php/dav/files/user/2024",
  "message": "Exception thrown: OCP\\Files\\NotFoundException",
  "userAgent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/137.0.0.0 Safari/537.36",
  "version": "30.0.14.1",
  "exception": {
    "Exception": "OCP\\Files\\NotFoundException",
    "Message": "/user/files_versions",
    "Code": 0,
    "Trace": [
      {
        "file": "/var/www/nextcloud/lib/private/Files/Node/LazyFolder.php",
        "line": 141,
        "function": "get",
        "class": "OC\\Files\\Node\\Root",
        "type": "->",
        "args": [
          "/user/files_versions"
        ]
      },
      {
        "file": "/var/www/nextcloud/apps/files_versions/lib/Versions/LegacyVersionsBackend.php",
        "line": 373,
        "function": "get",
        "class": "OC\\Files\\Node\\LazyFolder",
        "type": "->",
        "args": [
          "user/files_versions"
        ]
      },
...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[solracsf]

/backport to stable31

[solracsf]

/backport to stable30