State Machine: Use FIFO list for state traversal

Change to_visit from a set to a list and use pop(0) to perform a FIFO traversal of states. Replace set.add calls with list.append when enqueuing Next/Choices/Catch targets. This makes state traversal deterministic (BFS-like) instead of relying on unordered set.pop behavior while preserving reachable-state checks.

Author: r-heimann

src/cfnlint/rules/resources/stepfunctions/StateMachineDefinition.py

@@ -182,11 +182,11 @@ def _validate_states(
         # 2. Traverse reachable states from StartAt and collect errors for
         #    missing transition targets along the way.
         reachable: set[str] = set()
-        to_visit: set[str] = {start_at}
+        to_visit: list[str] = [start_at]
         transition_errors: list[ValidationError] = []
 
         while to_visit:
-            current = to_visit.pop()
+            current = to_visit.pop(0)
             if current in reachable or current not in states:
                 continue
 
@@ -210,7 +210,7 @@ def _validate_states(
                     [field],
                 )
                 if isinstance(target, str) and target in states:
-                    to_visit.add(target)
+                    to_visit.append(target)
 
             # Choices[].Next
             for idx, choice in enumerate(state.get("Choices") or []):
@@ -227,7 +227,7 @@ def _validate_states(
                     ["Choices", idx, "Next"],
                 )
                 if isinstance(target, str) and target in states:
-                    to_visit.add(target)
+                    to_visit.append(target)
 
             # Catch[].Next
             for idx, catch in enumerate(state.get("Catch") or []):
@@ -244,9 +244,7 @@ def _validate_states(
                     ["Catch", idx, "Next"],
                 )
                 if isinstance(target, str) and target in states:
-                    to_visit.add(target)
-
-            # Recurse into nested state machines (Parallel branches, Map processors)
+                    to_visit.append(target)
             state_type = state.get("Type")
 
             if state_type == "Parallel":