fix(workflow): detect duplicate trigger paths when first node has no id

validate_trigger_paths used seen_paths.get(trigger_path) and treated a None
result as "path not seen yet". But None is also what node.get("id") returns
for a node without an id, so when the first trigger node sharing a path had no
id, it was stored as None and every later node with the same path was silently
accepted as unique — duplicate trigger paths slipped through validation.

Use a membership test (trigger_path not in seen_paths) so "first occurrence"
and "node_id happens to be None" are no longer conflated. Behavior is
unchanged for nodes that have ids.

Adds a regression test that fails before and passes after.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

api/services/workflow/trigger_paths.py

@@ -127,8 +127,7 @@ def validate_trigger_paths(
                 )
             )
 
-        first_node_id = seen_paths.get(trigger_path)
-        if first_node_id is None:
+        if trigger_path not in seen_paths:
             seen_paths[trigger_path] = node_id
         else:
             issues.append(