ShellStep.validate() only checked that 'run' was present, so run: (null)
or a GitHub-Actions-style list validated clean; execute() then
str()-coerces the value and invokes it under shell=True, literally
running 'None' or "['echo', 'hi']" as a command. Add a type check after
the presence check, mirroring the command-step (#3262) and gate options
validation. Expression strings ('{{ ... }}') are strings, so they stay
valid.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>