feat(files): add WebDAV BDELETE for bulk deletions

[BarryOfficeEU]

Add a bounded BDELETE WebDAV method for deleting selected files in batches. Each target is deleted through the normal DAV unbind lifecycle without nested HTTP requests or mutation of the server request/response context.

Keep the feature behind bulk_delete.enabled and advertise the batch limit through capabilities so Files can fall back to individual DELETE requests when the feature is disabled or unavailable.

BDLETE-proof.mp4

Support Ticket#96104279
See also: #64611

Reviewer feedback from PR64611 addressed

1. API shape

The previous JSON POST /remote.php/dav/bulk-delete endpoint is removed. The implementation is now a WebDAV method handled by the DAV server.

2. DAV-shaped bulk deletion

The new method is BDELETE. Its request follows the Exchange-style structure:

<d:delete xmlns:d="DAV:">
   <d:target>
      <d:href>file1</d:href>
      <d:href>folder/file2</d:href>
   </d:target>

</d:delete>

The method is advertised through SabreDAV's getHTTPMethods() API. It returns 204 when every target succeeds and 207 Multi-Status when a target fails.

Reference supplied for review: Microsoft Exchange Server 2003 BDELETE documentation.

3. Request multiplexing

The old implementation created synthetic DELETE requests, changed the Sabre server's request/response/transaction state and re-entered invokeMethod() for every item. None of that remains.

BDELETE is one real DAV request. For each validated target it performs the normal unbind lifecycle directly:

1. emit beforeUnbind
2. delete through the DAV tree
3. emit afterUnbind

This keeps file permission checks, storage deletion and trash handling in the existing node/storage code without repeatedly invoking request-level plugins.

Assisted-by: ChatGPT:GPT-5.6-Sol

[BarryOfficeEU]

@icewind1991 here is an updated PR that addresses your review comments from #64611

[icewind1991]

Please use the existing sabre xml parsing/encoding methods instead

[icewind1991]

And this is still not following the AI policy: https://github.com/nextcloud/.github/blob/master/AI_POLICY.md