fix: guard against unregistered class_type in _is_intermediate_output

Commit 3696c5ba introduced _is_intermediate_output which iterates over
all node IDs in the prompt after execution, including orphan/disconnected
nodes that are never validated or executed. If any such node's class_type
is not in NODE_CLASS_MAPPINGS (e.g. a custom node pack that failed to
load or was renamed), NODE_CLASS_MAPPINGS[class_type] raises KeyError.

This exception short-circuits the while...else block, preventing:
- execution_success from being sent to the frontend
- self.history_result from being assigned

Images are still saved to disk since SaveImage runs during the main
execution loop, but the Gallery plugin never receives the success event
and the /history API returns incomplete data.

Fix: use .get() with a None guard so unregistered nodes are silently
treated as non-intermediate-output nodes.

execution.py

@@ -413,7 +413,9 @@ def format_value(x):
 
 def _is_intermediate_output(dynprompt, node_id):
     class_type = dynprompt.get_node(node_id)["class_type"]
-    class_def = nodes.NODE_CLASS_MAPPINGS[class_type]
+    class_def = nodes.NODE_CLASS_MAPPINGS.get(class_type)
+    if class_def is None:
+        return False
     return getattr(class_def, 'HAS_INTERMEDIATE_OUTPUT', False)
 
 def _send_cached_ui(server, node_id, display_node_id, cached, prompt_id, ui_outputs):