fix: update reactive ref after merge in imagePreviewStore (#8479)

## Summary
Fix for COM-14110: Preview image does not display new outputs in
vue-nodes.

## Problem
The merge logic in `setOutputsByLocatorId` updated `app.nodeOutputs` but
returned early without updating the reactive `nodeOutputs.value` ref.
This caused Vue components to never receive merged output updates
because only the non-reactive `app.nodeOutputs` was being updated.

## Solution
Added `nodeOutputs.value[nodeLocatorId] = existingOutput` after the
merge loop, before the return statement.

## Testing
- Added 2 unit tests covering the merge behavior
- All 4076 existing unit tests pass
- Typechecks pass
- Lint passes

## Notes
- Related open PRs touching same files: #8143, #8366 - potential minor
conflicts possible

Fixes COM-14110

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8479-fix-update-reactive-ref-after-merge-in-imagePreviewStore-2f86d73d365081f1a145fa5a9782515f)
by [Unito](https://www.unito.io)

Co-authored-by: Amp <amp@ampcode.com>

src/stores/imagePreviewStore.ts

@@ -148,6 +148,7 @@ export const useNodeOutputStore = defineStore('nodeOutput', () => {
             existingOutput[k] = newValue
           }
         }
+        nodeOutputs.value[nodeLocatorId] = existingOutput
         return
       }
     }