feat(objectstore): S3 conditional writes to prevent silent overwrites

[solracsf]

Summary

Nextcloud stores each file's data as an object keyed by the file-cache auto-increment id (urn:oid:<fileid>), so a newly created file always targets an object key that should not exist yet. When the file cache and the bucket drift out of sync, a DB restored from backup, two instances pointed at one bucket, or duplicated file ids, the current unconditional write
silently overwrites and destroys the existing object.

This adds opt-in S3 conditional writes: when enabled, creates send If-None-Match: * so the store atomically refuses to overwrite an existing object, turning silent data loss into a loud, data-preserving failure.

https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-writes.html

• S3 backend: If-None-Match: * on single-PUT and multipart creates; 412 typed exception, bounded 409 retry, abort-safe multipart completion.
• ObjectStoreStorage::writeStream uses it only for newly created files; on refusal it cleans up and logs a critical, actionable message (no data overwritten).
• New per-bucket conditional_writes config: false (default), 'auto' (one cached bucket probe, enable only if enforced), true. Self-disables on SigV2.

Compatibility

• Opt-in: default false, so upgrades are unchanged.
• Overwrites and the disabled path emit byte-identical S3 requests to before.
• No DB schema change.

Testing

• New unit tests: storage-layer routing + refusal-without-data-loss; S3 single/ multipart refusal + data preservation + config-off + cached-probe.
• Verified against MinIO

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 reviewed, and some tests written, by Fable

[solracsf]

1. Tests\Core\Sharing\Recipient\TeamShareRecipientTypeTest
   Circles app is not installed

PHPunit failure does not seem related.