fix: SaveImage node not updating outputs during batch runs (vue-nodes) (#8862)

## Summary

Fix vue-node outputs not updating during batch runs by creating a new
object reference on merge.

## Changes

- **What**: Spread merged output object in `setOutputsByLocatorId` so
Vue detects the assignment as a change. Adds regression test asserting
reference identity changes on merge.

## Review Focus

One-line fix at `imagePreviewStore.ts:155`: `{ ...existingOutput }`
instead of `existingOutput`. This matches the spread pattern already
used in `restoreOutputs` (line 368).

The root cause: Vue skips reactivity triggers for same-reference
assignments. The merge path mutated `existingOutput` in-place then
reassigned the same object, so `nodeMedia` computed in `LGraphNode.vue`
never re-evaluated.

> Notion:
https://www.notion.so/comfy-org/3066d73d36508165873fcbb9673dece7

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8862-fix-SaveImage-node-not-updating-outputs-during-batch-runs-vue-nodes-3076d73d36508133b1faeae66dcccf01)
by [Unito](https://www.unito.io)

Co-authored-by: Terry Jia <terryjia88@gmail.com>

src/stores/imagePreviewStore.ts

@@ -152,7 +152,7 @@ export const useNodeOutputStore = defineStore('nodeOutput', () => {
             existingOutput[k] = newValue
           }
         }
-        nodeOutputs.value[nodeLocatorId] = existingOutput
+        nodeOutputs.value[nodeLocatorId] = { ...existingOutput }
         return
       }
     }