Register cobra completion callbacks for all commands that accept
existing Velero resource names. A centralized completeNames helper
uses apimachinery's meta.ExtractList/Accessor to list resources with
a 3-second timeout, filter by prefix, and deduplicate already-typed
arguments. Wires ValidArgsFunction on 20 commands and
RegisterFlagCompletionFunc on 9 flags across backup, restore,
schedule, backuplocation, snapshotlocation, repo, and debug.
Closes#9782
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Joseph <jvaikath@redhat.com>