files_external amazons3: cross-mount MOVE should use server-side MultipartCopy when peer is same S3 endpoint

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Is your feature request related to a problem? Please describe.

\OC\Files\Storage\Common::copyFromStorage routes every cross-mount MOVE through PHP. When both mounts are files_external amazons3 on the same S3 endpoint, the source is read via fopen('r') and re-uploaded via \Aws\S3\MultipartUploader. The uploader's non-seekable branch materialises each part into php://temp before dispatch, so every part transits local temp storage. Temp usage scales with object size.

If the destination rejects a part, \Aws\Multipart\AbstractUploadManager::getResultHandler records the error and continues yielding parts. Temp usage grows unbounded.

Describe the solution you'd like

Add a fast path in Common::moveFromStorage (or an \OCA\Files_External\Lib\Storage\AmazonS3::copyFromStorage override). When source and destination are both AmazonS3 on the same endpoint, dispatch \Aws\S3\MultipartCopy (server-side UploadPartCopy with x-amz-copy-source).

Bytes stay on the S3 side. No PHP staging. Wall-clock reduces to seconds regardless of object size.

Precondition: the destination tenant's principal must hold s3:GetObject on the source bucket.

Describe alternatives you've considered

• Lower uploadPartSize and concurrency on the mount. Shrinks per-part footprint. Keeps O(N) buffering.
• Bucket-side LifecycleConfiguration AbortIncompleteMultipartUpload. Cleans up after the fact. Unsupported by NetApp StorageGrid.
• Out-of-band aws s3 copy. Bypasses Nextcloud. oc_filecache stays stale.
• #47986 (merged, NC 31). Same-bucket fast path inside ObjectStoreStorage. Cross-mount files_external remains uncovered.

Additional context

• \Aws\S3\MultipartCopy: https://docs.aws.amazon.com/aws-sdk-php/v3/api/class-Aws.S3.MultipartCopy.html
• UploadPartCopy IAM: https://docs.aws.amazon.com/AmazonS3/latest/API/API_UploadPartCopy.html
• Related open issues: #29841, #48194, #47856, #24540.

Start with Common::moveFromStorage and the files_external amazons3 storage implementation, then compare the same-bucket fast path from issue #47986. Confirm how source and destination mounts expose their S3 endpoints and how MultipartCopy is invoked. Done means same-endpoint cross-mount moves use server-side multipart copying without PHP temp staging while retaining Nextcloud file-cache behavior.