Function get_mutation_queue_key 

pub async fn get_mutation_queue_key(
    env: &dyn ExecutionEnv,
    path: &str,
    cancel: Option<&CancellationToken>,
) -> Result<String, FileError>

Compute the canonical-path queue key for path. Mirrors getMutationQueueKey:

1. absolutePath(path) → propagate err.
2. canonicalPath(absolute):
   • Ok → return canonical string.
   • Err with not_found or not_supported → fall back to absolute string.
   • other err → propagate.