chore(infra): add openai back to core release test matrix (#34372)

Reverts langchain-ai/langchain#34020

.github/workflows/_release.yml

@@ -396,7 +396,7 @@ jobs:
       contents: read
     strategy:
       matrix:
-        partner: [anthropic]
+        partner: [openai, anthropic]
       fail-fast: false # Continue testing other partners if one fails
     env:
       ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}