`apx stacks new` and `apx stacks update` both have a strict limit for packages list

Start by reproducing the package entry prompts in apx stacks new and apx stacks update with the 183-character package list shown in the issue, then trace where the input length is validated. Done means both commands accept the reported package list without truncating it, while existing package input behavior remains intact.

When I create a stack or update an existed stack, and edit the packages list, I find that the packages list has a strict length limit.

My expected packages list is:

build-essential cmake git rust-all python3 python3-venv python3-dev python3-sympy python3-numpy python3-scipy python3-matplotlib python3-h5py python3-serial libeigen3-dev texlive-full

seperated by space as required, 15 packages, and 183 char in total.

When creating or updating a stack:

lie@apx-vso-native:~$ apx stacks --help
Usage: stacks [flags] [command]

Work with the stacks that are available in apx.

Commands:
  export          Export the specified stack.
  import          Import the specified stack.
  list            List all available stacks.
  new             Create a new stack.
  rm              Remove the specified stack.
  show            Show information about the specified stack.
  update          Update the specified stack.

lie@apx-vso-native:~$ apx stacks new
Choose a name:
➜ test_demo                                                                                                                                                                                                 
Choose a base (e.g. 'vanillaos/pico'):
➜ ubuntu:latest                                                                                                                                                                                             

    Select a package manager [1-5]:    
                                       
    1. apk                             
  > 2. apt                             
    3. dnf                             
    4. pacman                          
    5. zypper                          
                                       
                                       
                                       
                                       
                                       
    ↑/k up • ↓/j down • q quit • ? more

You have not provided any packages to install in the stack. Do you want to add some now?

y  N  
Please type the packages you want to install in the stack, separated by a space:
➜ build-essential cmake git rust-all python3 python3-venv python3-dev python3-sympy python3-numpy python3-scipy python3-matplotlib python3-h5py python3-serial                                              
INF > Created stack 'test_demo'.
lie@apx-vso-native:~$ apx stacks update test_demo
Type a new base or confirm the current one (ubuntu:latest):
➜ ubuntu:latest                                                                                                                                                                                             
Choose a new package manager or confirm the current one (apt):
➜ apt                                                                                                                                                                                                       

Do you want to update the list of packages?
     -build-essential                      
     - cmake                               
     - git                                 
     - rust-all                            
     - python3                             
     - python3-venv                        
     - python3-dev                         
     - python3-sympy                       
     - python3-numpy                       
     - python3-scipy                       
     - python3-matplotlib                  
     - python3-h5py                        
     - python3-serial                      

y  N  
Type the packages you want to install in the stack, separated by a space:
➜ build-essential cmake git rust-all python3 python3-venv python3-dev python3-sympy python3-numpy python3-scipy python3-matplotlib python3-h5py python3-serial                                              
INF > Updated stack 'test_demo'.

Only 156 chars allowed.