fix(plan): update robot helper to assert alternatives key in explain output

The robot/helper_plan_explain.py integration test helper was still
asserting the old field name alternatives_considered in the explain
dict output. Since _build_explain_dict() now outputs alternatives
(structured objects with index/description/chosen fields per spec),
the assertion was failing the CI integration_tests job.

Updated the assertion to check for alternatives and also verify
it is a list, matching the new structured output format.

robot/helper_plan_explain.py

@@ -49,7 +49,8 @@ def _test_explain_format() -> None:
     assert "decision_id" in data
     assert "context_snapshot" in data
     assert "rationale" in data
-    assert "alternatives_considered" in data
+    assert "alternatives" in data
+    assert isinstance(data["alternatives"], list)
     assert data["question"] == "What to build?"
     print("plan-explain-ok")