fix(files_external): propagate child copy failures in AmazonS3::copy()

[rnixx]

Summary

When copying a directory, copy() discarded the return values of its recursive calls and always returned true. rename() relies on that value, so a failed copy still led to rmdir() on the source: every file in the directory was deleted, the destination stayed empty, and the UI reported success.

This is reachable whenever a provider rejects CopyObject for objects it otherwise serves — Hetzner Object Storage answers 501 NotImplemented for SSE-C encrypted objects, which makes every single child copy fail.

Behaviour with the fix (measured on 34.0.1, Hetzner Object Storage + SSE-C)

Renaming a folder now fails visibly instead of destroying its contents:

• Web UI: the server answers 500, the files stay where they are. The
  destination directory created by mkdir() before the failure remains as an
  empty folder and can be deleted normally.
• Desktop client: the MOVE fails with 500, and the client rolls the
  local rename back and re-fetches the files. It does not fall back to
  upload+delete, so nothing is duplicated and nothing is lost.
• Upload, download and delete are unaffected.

Renaming remains impossible on such providers — that part is outside
Nextcloud's control. What changes is that the failure is now visible instead
of silently deleting the data.

Related

Previously reported without an issue:

https://help.nextcloud.com/t/nc-copy-move-not-working-and-destroying-files-in-s3-external-storage-with-sse-c-key/231896

(NC 31.0.8, same provider, same 501 NotImplemented, same silent data loss)

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)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[rnixx]

Thanks for the review — comments shortened as suggested.

For the test setup I could only apply invokePrivate() to two of the four
lines; the other two run into limitations of the helper:

• 'test': AmazonS3::test() exists as a public method, and invokePrivate()
  checks hasMethod() before hasProperty(). It calls the method instead of
  setting the property, failing with
  Typed property AmazonS3::$params must not be accessed before initialization.
• 'logger': the property is private on AmazonS3, so it is invisible on the
  PHPUnit mock subclass that invokePrivate() reflects on — hasProperty()
  returns false and the helper silently sets nothing.

initCaches and storageClass work and now use the helper. The remaining two
keep ReflectionProperty with a short comment explaining why.

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22

[github-actions]

Hello there,
Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!

(If you believe you should not receive this message, you can add yourself to the blocklist.)