fix(workflows): fail fan-out loudly on a truthy non-mapping step template (#3537)

* fix(workflows): fail fan-out loudly on a truthy non-mapping step template

A fan-out step whose `step:` is a truthy scalar or list (an authoring mistake) passed execute and reached the engine, which calls template.get("id", ...) in _run_fan_out — raising AttributeError and taking down the whole run. validate already rejects a non-mapping step, but the engine does not auto-validate, so an unvalidated run crashed.

Guard execute to FAIL the step (with a clear error and normalized empty output) instead, mirroring the existing non-list items guard and the switch non-dict cases guard. Add the matching test_execute_non_dict_step_fails_loudly covering the execute-path guard (validate was already covered).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(workflows): reject explicit fan-out `step: null` in validate()

The runtime guard in execute() rejects a truthy non-mapping step, but
`config.get("step", {})` only substitutes the `{}` default for an *absent*
key — an explicit `step: null` reaches the guard as None and FAILS the step.
validate() previously exempted None (`step is not None and ...`), so such a
workflow passed validation and then failed during execution.

Align validate() with the runtime guard: a present-but-non-mapping `step`
(including `None`) is an authoring mistake and is now rejected up front.
Extend the validate and execute regression cases to cover None.

Addresses Copilot review feedback on #3537.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/specify_cli/workflows/steps/fan_out/__init__.py

@@ -25,6 +25,33 @@ class FanOutStep(StepBase):
         max_concurrency = config.get("max_concurrency", 1)
         step_template = config.get("step", {})
 
+        # The engine does not auto-validate step config (see
+        # ``WorkflowEngine.load_workflow``). On a COMPLETED fan-out it reads the
+        # ``step_template`` back out and, when it is truthy, calls
+        # ``template.get("id", ...)`` in ``_run_fan_out``. A truthy non-mapping
+        # ``step`` (a scalar or list authoring mistake) would crash the whole
+        # run with AttributeError there — the engine invokes ``execute`` and
+        # ``_run_fan_out`` with no surrounding try/except. ``validate`` already
+        # rejects a non-mapping ``step``; fail this step loudly on an
+        # unvalidated run instead, mirroring the ``items`` guard below. An empty
+        # or absent ``step`` defaults to ``{}`` (falsy) and the engine's
+        # ``if template and items`` skips fan-out, so it stays valid here.
+        if not isinstance(step_template, dict):
+            return StepResult(
+                status=StepStatus.FAILED,
+                error=(
+                    f"Fan-out step {config.get('id', '?')!r}: 'step' must be a "
+                    f"mapping (nested step template), got "
+                    f"{type(step_template).__name__}."
+                ),
+                output={
+                    "items": [],
+                    "max_concurrency": max_concurrency,
+                    "step_template": {},
+                    "item_count": 0,
+                },
+            )
+
         if not isinstance(items, list):
             # A non-list here is a wiring error (the expression did not
             # resolve to a collection); silently fanning out over zero
@@ -66,8 +93,13 @@ class FanOutStep(StepBase):
                 f"Fan-out step {config.get('id', '?')!r} is missing "
                 f"'step' field (nested step template)."
             )
-        step = config.get("step")
-        if step is not None and not isinstance(step, dict):
+        elif not isinstance(config["step"], dict):
+            # A present-but-non-mapping ``step`` (including an explicit
+            # ``step: null``) is an authoring mistake. ``config.get("step", {})``
+            # in ``execute`` only substitutes the ``{}`` default for an *absent*
+            # key, so an explicit ``None`` reaches the runtime guard and FAILS
+            # the step. Reject it here too so a workflow cannot pass validation
+            # and then fail during execution.
             errors.append(
                 f"Fan-out step {config.get('id', '?')!r}: 'step' must be a mapping."
             )