[EXP][CMDBUF] Make command handle behaviour consistent

[Bensuo]

• Attempting to obtain a command handle when the command buffer is not updatable is now an error across all adapters
• command-buffer spec and adapter code updated to reflect this
• invalid_update CTS test updated to reflect new behaviour

[Bensuo]

DPC++ PR: https://github.com/intel/llvm/pull/15522

[aarongreig]

looking at merging this soon if you can fix the conflicts @Bensuo

[Bensuo]

looking at merging this soon if you can fix the conflicts @Bensuo

Thanks! I've updated the branch and will keep an eye on the CI while it runs.