* fix(workflows): reject a fan-in wait_for that names an unknown step at validation
* fix(workflows): reject fan-in wait_for self-reference and non-string entries
Address review feedback on the fan-in wait_for validator:
- A fan-in's own id is added to seen_ids before the wait_for check, so
`wait_for: [<self>]` passed validation while producing a silent empty
join at runtime. Reject self-references explicitly.
- Non-string entries (e.g. YAML `wait_for: [123]`) were skipped by the
isinstance(str) guard and validated even though they can never match a
real step id. Flag them as wiring errors.
Add coverage for both cases.
3036fe6954