Function validate_and_run_command 

pub async fn validate_and_run_command(
    backend: &Arc<dyn ExternalBackend>,
    gh: &GhHttp,
    repo: &str,
    issue_number: &str,
    command: &OwnerCommand,
    author: &str,
    store: &Option<Arc<dyn CommandStoreOps>>,
    task_manager: &Arc<TaskManager>,
) -> CommandOutcome

Validate a slash command (issue open, author is collaborator) and execute it.

Shared by both scan_commands (issue comments) and scan_mentions (mention handling). Posts a confirmation or error comment on the issue/PR.