Install, build, and start commands saved before they were validated
blocked every save of the General tab, and the fields are hidden for
build packs other than Nixpacks and Railpack, so users could not fix
them. Validate these commands only when they change; deployments pass
them to the builder as one escaped argument.
Closes#10055
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
