fix(workflows): require a steps body on while and do-while loops

[jawwad-ali]

Problem

Both loop steps type-check steps ('steps' must be a list) but never require it to be present, so an absent body silently becomes [].

Every sibling control-flow step already requires its body:

step	required body key	enforced
if	then	✅ if_then/__init__.py:84
fan-out	items + step	✅ fan_out/__init__.py:87, 92
while	steps	❌
do-while	steps	❌

The mistype is unusually easy here: the fan-out step's own payload key is the singular step: while the loops use steps:.

Reproduction on current main

A. while, body key typo'd as singular step:
   validate: []
   execute : StepStatus.COMPLETED | next_steps = []
B. do-while, no steps at all:
   validate: []
   execute : StepStatus.COMPLETED | next_steps = []

specify workflow validate reports zero errors. At run time the step reports COMPLETED while returning no next_steps, so the engine's if result.next_steps: block never fires and the retry loop the workflow is built around never runs even once — a silent no-op with no diagnostic anywhere.

For do-while this also contradicts the step's own docstring: "The first invocation always returns the nested steps for execution."

Fix

Add the missing presence check to both validate() methods, worded to match the existing if step's message.

Behaviour change — disclosed

A workflow that today declares a while/do-while with no steps key now fails specify workflow validate instead of passing. That is the point of the fix: such a loop is already a guaranteed no-op, so the change converts a silent misconfiguration into a clear error rather than altering any working behaviour. Runtime execution is untouched.

I scanned every workflow-shaped YAML in the repo — 0 bodyless loops, so no in-tree workflow or template is affected. An explicitly empty steps: [] still validates exactly as before.

Verification

• Fail-before / pass-after: with both source files reverted to upstream/main, the 2 new tests fail → 37 passed with the fix.
• Scoped regression across the loop/validate/engine surface (tests/test_workflows.py + tests/workflows, -k "While or DoWhile or Loop or loop or Validat or validate or Engine"): 283 passed.
• Both existing test_validate_missing_fields tests already pass "steps": [], so they are unaffected.
• Each new test also asserts the consequence — that execution is a silent no-op — not just the message.
• uvx ruff@0.15.0 check src tests → clean

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Written with assistance from Claude Code. Bug found, reproduced, and verified by me on current main.