fix: prevent TypeError when nodeDef is undefined in NodeTooltip (#5659)

## Summary

Fix TypeError in NodeTooltip component when `nodeDef` is undefined. This
occurs when hovering over nodes whose type is not found in the
nodeDefStore.

## Changes

- Add optional chaining (`?.`) to `nodeDef.description` access on line
71
- Follows the same defensive pattern used in previous fixes for similar
issues

## Context

This addresses Sentry issue
[CLOUD-FRONTEND-STAGING-1B](https://comfy-org.sentry.io/issues/6829258525/)
which shows 19 occurrences affecting 14 users.

The fix follows the same pattern as previous commits:
-
[290bf52fc](290bf52fc5)
- Fixed similar issue on line 112
-
[e8997a765](e8997a7653)
- Fixed multiple similar issues


┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5659-fix-prevent-TypeError-when-nodeDef-is-undefined-in-NodeTooltip-2736d73d3650816e8be3f44889198b58)
by [Unito](https://www.unito.io)

src/components/graph/NodeTooltip.vue

@@ -68,7 +68,7 @@ const onIdle = () => {
     ctor.title_mode !== LiteGraph.NO_TITLE &&
     canvas.graph_mouse[1] < node.pos[1] // If we are over a node, but not within the node then we are on its title
   ) {
-    return showTooltip(nodeDef.description)
+    return showTooltip(nodeDef?.description)
   }
 
   if (node.flags?.collapsed) return