`GateStep.validate` rejects a non-list (or empty) `options` and requires
every option to be a string, but the engine does not auto-validate before
`execute`. On an unvalidated run a scalar/dict/None `options` reached
`_prompt` and crashed the whole workflow with a raw `TypeError`
(`enumerate`/`len` on a non-iterable) or `KeyError` (indexing a dict); an
empty list spun `_prompt`'s input loop forever; a non-string option crashed
the reject check at `choice.lower()` with `AttributeError`.
Guard `execute` to FAIL the step cleanly instead, before the non-TTY
PAUSE short-circuit so the error surfaces in CI too rather than pausing
and only crashing later on interactive resume. Mirrors the switch 'cases'
and command 'input' unvalidated-execute guards.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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