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.