fix(workflows): if step execute() fails cleanly on a non-list branch

[jawwad-ali]

What

IfThenStep.execute returns config['then']/['else'] directly as StepResult.next_steps with no check that it's a list:

branch = config.get("then", [])
return StepResult(status=COMPLETED, next_steps=branch)   # branch could be "oops" / {...} / 42

The engine does not auto-validate step config before execute(), so an unvalidated run with a non-list branch (a scalar or mapping authoring mistake) passes a non-iterable as next_steps and crashes the whole run. validate() catches this, but execute() should fail the step loudly rather than take down the run.

Fix

Add an isinstance(..., list) guard returning a FAILED StepResult. This completes the same guard #3519 added to the while/do-while steps — whose comment already references the if step as a peer — and mirrors the switch step's cases guard.

Note: this PR originally also touched while/do-while, but #3519 landed that guard for those two steps in the meantime. Rebased down to the still-missing if case only, to avoid duplicating merged work.

Test

Parametrized test_execute_rejects_non_list_branch feeds a str/dict/int branch and asserts FAILED — fails before (the non-iterable next_steps crashed execution), passes after.

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🤖 Written with the assistance of Claude Code (AI). Bug self-found; fix/tests verified locally (fail-before / pass-after), ruff clean.

[jawwad-ali]

Heads-up: since opening this, #3519 landed the same non-list guard for the while/do-while steps. I've rebased this PR down to the one case #3519 didn't cover — the if step's execute() — so it now complements #3519 rather than duplicating it (and the earlier merge conflict is resolved).