[PB-5606]: refactor/update function and existance check

[jzunigax2]

Optimize file uploads with targeted existence check and rename-based updates

Previously:

• Update() called NewObject() to check file existence
• NewObject() used folders.ListAllFiles() - fetches ALL files in directory to find one matching file
• Inefficient for directories with many files
• Delete-then-upload pattern: old file deleted before new upload starts
• If upload fails, data is lost

Optimizations

1. Targeted Existence Check

• New preUploadCheck() uses files.CheckFilesExistence() API
• Checks for specific file by name instead of listing entire directory, plus previous approach was unreliable in the case the response had to be paginated

2. Rename-Based Atomic Updates

• Rename existing file to backup with random suffix
• Upload new file with original name
• On success: delete backup; on failure: restore backup
• Guarantees data safety during upload failures