build: migrate release workflow to upstream release-please

[jbeckwith-oai]

Summary

Migrate the release workflow from stainless-api/trigger-release-please to the standard googleapis/release-please-action.

What changed

• switched .github/workflows/create-releases.yml to manifest-mode googleapis/release-please-action
• pinned the action to the latest upstream commit SHA we verified during the migration
• split the workflow into separate release and publish jobs so broader release-please permissions are not granted during publish
• updated the publish job guard to still run when release_created is true even if the release job later reports failure
• updated release-please-config.json to point at the upstream googleapis/release-please schema

Why

• removes the dependency on the Stainless-specific wrapper action
• aligns this repo with the standard upstream release-please setup
• preserves the existing release-PR model while tightening permission scope in the publish path
• avoids a partial-failure case where a GitHub release could be created but npm/JSR publish would be skipped

Validation

• validated .github/workflows/create-releases.yml parses as YAML
• validated release-please-config.json parses as JSON
• validated the config against the upstream release-please JSON schema
• reviewed npm publish auth flow: the workflow is set up to use GitHub Actions OIDC under the hood via id-token: write; repo-side workflow code does not require an NPM_TOKEN in Actions

Notes

• the workflow currently uses github.token for release-please, not a PAT
• direct pushes to openai/openai-node workflow files are blocked by repo/org rulesets, so this PR is opened from a fork