fix(step): iterate a scalar foreach value once instead of crashing on falsy values

[yzxcj797]

Fixes #6721.

Summary

Step._get_foreach_items decided between "iterate a single resolved reference" and "zip() multiple &&-combined references" with the classic X and Y or Z idiom:

return len(foreach_items) == 1 and foreach_items[0] or zip(*foreach_items)

The idiom is only safe when Y is never falsy. A single reference that resolves to 0, False or None — e.g. foreach: "{{ steps.check-count.results }}" where the count is zero — fell through to zip(*foreach_items) = zip(0), raising TypeError: 'int' object is not iterable. A truthy non-iterable scalar was returned bare and crashed the same way one line later in _run_foreach's for item in items.

The fix wraps a non-iterable single value in a list so the action runs exactly once with the resolved value:

if len(foreach_items) == 1:
    value = foreach_items[0]
    if isinstance(value, Iterable):
        return value
    return [value]
return zip(*foreach_items)

Iterables (lists, dicts, strings) keep their existing semantics and the multi-reference zip path is untouched.

Testing

Added test_get_foreach_items_single_reference (parametrized over 0, False, None, 5, lists and the empty list) and test_get_foreach_items_multiple_references_zip to tests/test_steps.py, next to the existing Step unit tests with a mocked context manager.

Behavior differential of the expression (old vs new, values materialized):

single resolved value	old	new
0 / False / None	TypeError from zip(...)	[value] (one iteration)
5	5 (then TypeError in the consumer's for-loop)	[5] (one iteration)
[1, 2], [], other iterables	unchanged	unchanged
multiple refs	zip(...)	unchanged

Note: I could not run the full tests/test_steps.py suite locally — the project requires Python ≥ 3.11 (local interpreter is 3.10) and the conftest import chain needs the full API environment. The added tests follow the existing unit-test pattern in the same file and run under the project's CI.

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