CodeMode again blocks shell access on IPython 9.17

We fixed CodeMode(allow_shell=False) on IPython 9.17. It once again blocks shell execution, so shell magics like %%bash cannot run in a kernel you have told to forbid them.

IPython 9.17 recreates script magics whenever something looks them up, which let %%bash keep running even with allow_shell=False. We now strip those magics after the kernel recreates them, so your setting holds.