Prevent kubectl-us backups from overwriting on filename collisions

[cardoe]

Problem

The backup command introduced by #2344 names backup files using the Kubernetes context, backup type, and int(time.time()). Because the timestamp has one-second resolution, concurrent invocations for the same context can choose the same destination filename.

The successful write path uses os.replace(tmp_path, dest), which replaces an existing destination. A later successful backup can therefore silently overwrite an earlier valid backup, despite the code comment stating that an existing same-name backup is never clobbered.

Expected behavior

A successful backup must not overwrite an existing backup file. The command should either generate collision-resistant filenames or fail safely when the destination has already been reserved or exists.

Acceptance criteria

• Existing completed backups are never silently overwritten.
• Concurrent invocations cannot claim the same output filename.
• Tests cover a successful destination-name collision.

Context

Found while reviewing #2344.