fix: dispatch cloud build on synchronize for preview-labeled PRs (#9636)

## Summary

Cloud build dispatch was only triggering on the `labeled` event, not on
subsequent pushes to PRs that already had a preview label.

## Changes

- **What**: Add `synchronize` to `pull_request` event types and update
the `if` condition to support all three preview labels (`preview`,
`preview-cpu`, `preview-gpu`). For `labeled` events, check the added
label name; for `synchronize` events, check existing PR labels.

## Review Focus

The `if` condition now branches on `github.event.action` to use the
correct label-checking mechanism for each event type.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9636-fix-dispatch-cloud-build-on-synchronize-for-preview-labeled-PRs-31e6d73d3650814e9069e37d6199ffc9)
by [Unito](https://www.unito.io)

.github/workflows/cloud-dispatch-build.yaml

@@ -14,7 +14,7 @@ on:
       - 'cloud/*'
       - 'main'
   pull_request:
-    types: [labeled]
+    types: [labeled, synchronize]
   workflow_dispatch:
 
 permissions: {}
@@ -26,11 +26,18 @@ concurrency:
 jobs:
   dispatch:
     # Fork guard: prevent forks from dispatching to the cloud repo.
-    # For pull_request events, only dispatch when the 'preview' label is added.
+    # For pull_request events, only dispatch for preview labels.
+    # - labeled: fires when a label is added; check the added label name.
+    # - synchronize: fires on push; check existing labels on the PR.
     if: >
       github.repository == 'Comfy-Org/ComfyUI_frontend' &&
       (github.event_name != 'pull_request' ||
-       github.event.label.name == 'preview')
+       (github.event.action == 'labeled' &&
+        contains(fromJSON('["preview","preview-cpu","preview-gpu"]'), github.event.label.name)) ||
+       (github.event.action == 'synchronize' &&
+        (contains(github.event.pull_request.labels.*.name, 'preview') ||
+         contains(github.event.pull_request.labels.*.name, 'preview-cpu') ||
+         contains(github.event.pull_request.labels.*.name, 'preview-gpu'))))
     runs-on: ubuntu-latest
     steps:
       - name: Build client payload