`SwitchStep.validate()` already rejects a non-mapping `cases`, but the
engine's `execute()` path does not auto-validate (see
`WorkflowEngine.load_workflow`, whose docstring notes the definition is
"not yet validated"). On an unvalidated run, `execute` called
`cases.items()` on the raw value, so a list or scalar `cases` authoring
mistake raised `AttributeError` and took down the whole run — the engine
invokes `step_impl.execute()` with no surrounding try/except.
Guard `execute` to return a FAILED StepResult naming the type error
instead, mirroring the fan-out step's non-list `items` handling. The
expression is still evaluated first, so its value is surfaced in the
step output for downstream context.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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