fix(libsync): add NEXTCLOUD_BULK_UPLOAD env override to force-disable bulk upload

[mosandlt]

Summary

Adds an OWNCLOUD_BULK_UPLOAD environment variable override to Capabilities::bulkUpload(), mirroring the existing OWNCLOUD_CHUNKING_NG pattern in the same file (src/libsync/capabilities.cpp).

Problem

Bulk upload (POST .../dav/bulk) is enabled automatically whenever the server advertises the dav.bulkupload capability, with no client-side way to opt out if that endpoint is broken or blocked somewhere in the network path (reverse proxy, WAF, CDN, etc).

Observed against a self-hosted Nextcloud instance where every single bulk-upload POST failed immediately with UnknownNetworkError, while regular per-file uploads (PropagateUploadFileNG) succeeded without issue. Because the failure is deterministic and the in-memory per-file bulk-upload blacklist does not survive across the --max-sync-retries internal restarts (let alone separate nextcloudcmd invocations from a cron/systemd timer), affected files never converge — the sync stays permanently red for every file it decides to bulk-upload, run after run, with no recovery path short of patching the client.

Fix

OWNCLOUD_BULK_UPLOAD=0 forces bulk upload off regardless of server capabilities (falls back to the normal single-file upload path). OWNCLOUD_BULK_UPLOAD=1 forces it on. Unset preserves current behavior exactly (server capability decides).

Verified locally: rebuilding nextcloudcmd/libnextcloudsync with this change and setting OWNCLOUD_BULK_UPLOAD=0 took a sync from a reproducible ~630-file failure batch (100% of that sync's bulk-upload attempts failing with UnknownNetworkError, identical set of files failing across 3 consecutive runs) down to a clean run with zero bulk-related errors, using the regular upload path instead.

Testing

• Local dpkg-buildpackage build against Debian 12 (bookworm) package 3.7.3-1+deb12u2
• Confirmed OWNCLOUD_BULK_UPLOAD=0 / =1 / unset all behave as expected against a real server
• No existing tests cover Capabilities::bulkUpload(); behavior for the unset case is unchanged (same expression as before, now reached only after the two new early-return checks)

[mgallien]

thanks

[mgallien]

@mosandlt gentle reminder
you might want to finish the PR such that we can merge it

[mosandlt]

Renamed to NEXTCLOUD_BULK_UPLOAD per your suggestion, thanks!

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-10300.zip

Digest: sha256:f1c72ae991d1ef8a845e9322afb4f76ae799967e6325ad82275d7d0ce345d1cf

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

Please make sure to quit your existing Nextcloud app and backup your data.

[github-actions]

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