ci(infra): gate PyPI publish on prior-partner tests (#37923)

The `🚀 Publish to PyPI` job no longer starts until `🔄 Test prior
partners against new core` finishes. Previously that dependency was
commented out, so a core release could publish to PyPI in parallel
with—or before—the integration tests that install the new unreleased
core against already-published partner packages, defeating their purpose
as a pre-publish gate.

## Changes
- Add `test-prior-published-packages-against-new-core` to the `publish`
job's `needs`, so publishing blocks on those partner integration tests
completing.
- The existing `if: ${{ !cancelled() && !failure() }}` guard is
unchanged: publish proceeds only if the gate **succeeded or was
skipped**, and fails closed if the partner tests fail. For non-core
releases the partner-test job short-circuits with `exit 0`, so this adds
no friction outside `libs/core` releases.

.github/workflows/_release.yml

@@ -638,8 +638,9 @@ jobs:
       - test-pypi-publish
       - pre-release-checks
       - test-dependents
-      # - test-prior-published-packages-against-new-core
+      - test-prior-published-packages-against-new-core
-      # Run if all needed jobs succeeded or were skipped (test-dependents only runs for core/langchain_v1)
+      # Run if all needed jobs succeeded or were skipped (test-dependents and
+      # test-prior-published-packages-against-new-core only run for core/langchain_v1)
     if: ${{ !cancelled() && !failure() }}
     environment: Release
     runs-on: ubuntu-latest