Function validate_replace_request 

async fn validate_replace_request(
    state: &AppState,
    req: ReplaceOrderRequest,
    index: usize,
) -> Result<ValidatedReplace, BulkOrderResult>

Run signature verification, agent authorization, symbol parsing, trading-allowed checks, and price/size parsing + precision checks. Returns a ValidatedReplace on success, or a pre-populated BulkOrderResult carrying the rejection reason on any failure.