`FanInStep.validate()` and the engine's fan-in checks both reject a
non-list `wait_for`, 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`
iterated the raw value with `for step_id in wait_for`, with two bad
outcomes:
* a scalar (`wait_for: 5`, `wait_for: null`) raised `TypeError` and
took down the whole run — the engine invokes `step_impl.execute()`
with no surrounding try/except; and
* a string (`wait_for: stepA`) silently iterated its characters and
returned a join of empty results with a COMPLETED status — the exact
"silent empty result + COMPLETED" wiring bug the engine's own fan-in
validation comment warns against.
Guard `execute` to return a FAILED StepResult naming the type error
instead, mirroring the fan-out step's non-list `items` handling. A
missing `wait_for` key still defaults to an empty list (COMPLETED),
unchanged; the guard fires only on an explicit non-list value.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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