Ensure workflow is triggered on pull request updates (#2)

* Ensure workflow is triggered on pull request update

Ensure `synchronize` event also trigger the workflow, as relying solely on push events failed for pull requests from forked repositories.

See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request

* Prevent multiple triggers of the workflow on pull request

Push triggers outside of pull requests are omitted intentionally to prevent duplicate workflow runs. Since this repository mainly involves managing service declarations rather than altering software features, running tests on every commit of every branch isn't seen as essential compared to traditional software.

.github/workflows/test.yml

@@ -1,11 +1,7 @@
 name: Validate declarations
 
 on:
-  push:
-    branches-ignore:
-      - 'main'
   pull_request:
-    types: [ opened, reopened ]
   workflow_call:
 
 jobs: