perf(dav): Optimize system tag permission checks for multi-mount file IDs

[joshtrichards]

Summary

Optimize SystemTagPlugin::canUpdateTagForFileIds() by using getFirstNodeById() as a fast path and falling back to getById() only when needed.

A file ID can be visible through multiple mounts or shares with different effective permissions. This change keeps the existing batch behavior that all requested file IDs must be authorized, while treating a file ID as updateable when at least one visible occurrence has PERMISSION_UPDATE.

• Aligns the permission check with similar logic in related DAV/system tag code
• Preserves correctness for multi-mount / multi-share edge cases
• Reduces work in the common case

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 partly or fully generated using AI