Improved upload chunk integrity

[i2h3]

Fixes a chunked-upload integrity bug in the macOS File Provider extension that affects large files (>100 MiB, e.g. Adobe InDesign/Illustrator documents). Previously the chunk-upload identifier diverged across a file's lifecycle — setStatusForItemMetadata minted a fresh UUID on every .uploading transition while modifyContents passed a stale, pre-transition value to the uploader — so an interrupted transfer never actually resumed (it re-uploaded from scratch and leaked both local chunk records and the server-side upload folder), and because chunks carried no binding to the content they came from, a resumed upload whose source had since changed could splice stale server-side chunks into the new version and silently produce a corrupted file. The chunk identifier is now derived deterministically from (itemIdentifier, fileSize, modificationDate): identical content re-uploads under the same id so an interrupted transfer correctly resumes and reuses the chunks already stored, while any content change yields a new id (and a fresh server folder), guaranteeing old chunks can never be mixed into a different version; the item's stale chunk bookkeeping is swept before each upload. The now-redundant chunkUploadId churn was removed from setStatusForItemMetadata (the Realm field is left in place, so there is no schema migration). This is covered by the resume tests for the create and modify paths (rewritten for the derived id) plus a new test asserting that a content change discards the prior chunks and starts fresh; the full test suite (315 tests) passes.

Checklist

• Sign-off message is added to all commits.
• The commit history is clean with no merge commits.
  • How to rebase your commits
  • How to squash commits with rebase
• Uploaded screenshots from before and after for UI changes.
• Documentation has been updated or is not required.
• Backports requested where applicable (critical bugfixes).
• Labels added where applicable (bug/enhancement).
• Milestone added for target version.

AI (if applicable)

• The content of this PR was partly or fully generated using AI
  • I have read the guidelines for AI-assisted contributions.
  • I have read Nextcloud's AI Contribution Policy.

[i2h3]

/backport to stable-34.0

[i2h3]

This depends on #10369 because it also fixes tests which are flaky master-wide.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-10377.zip

Digest: sha256:518e397798e50640a3d378fc9c23ef9fe9d33f1c076fd78b2e53c43d814b3d8e

To test this change/fix you can download the above artifact file, unzip it, and run it.

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