Fix "Generate API docs JSON" workflow failing on pull_request events

[Copilot]

On pull_request events, github.ref resolves to refs/pull/<N>/merge — a virtual ref that actions/checkout cannot fetch directly — causing the workflow to fail with fatal: couldn't find remote ref.

Changes

• generate-api-docs.yml: Added a dedicated pull_request branch in the "Get ref" step that uses github.sha (the actual merge commit SHA) instead of github.ref, which is only valid for tag/branch push events.

- else
-   echo "Get ref from push"
-   echo "ENGINE_REF=${{ github.ref }}" >> $GITHUB_OUTPUT
+ elif [ "${{ github.event_name }}" = 'pull_request' ]; then
+   echo "Get ref from pull_request"
+   echo "ENGINE_REF=${{ github.sha }}" >> $GITHUB_OUTPUT
+ else
+   echo "Get ref from push"
+   echo "ENGINE_REF=${{ github.ref }}" >> $GITHUB_OUTPUT