[codex] Wait for stable PGP keyserver visibility

[wk1995]

Root cause

The previous fix uploaded the PGP public key before publishing, but run 28764080059 still failed because Central checked for the signature key before the key was consistently visible from the supported keyserver network.

Evidence from the failed run:

• Ensure PGP public key is available succeeded for BEBA3624842B5A7925478B4A00BDDFF01651B51F.
• Publish to Central staging succeeded.
• Create Central Portal deployment failed immediately after with Could not find a public key by the key fingerprint.
• External fresh-keyring lookups against keyserver.ubuntu.com were inconsistent right after the run, showing keyserver visibility was not stable.

Summary

• Require multiple consecutive fresh-keyring lookups before considering the public key available.
• Move the public-key availability step before JDK/Gradle setup so the build and local validation time also gives the keyserver network time to settle.
• Keep Central credentials and signing password scoped out of the public-key preflight step.

Validation

• python3 -m unittest discover -s .github/scripts -p '*_test.py'
• ./gradlew :plugin_base:build --stacktrace
• ./gradlew :plugin_base:publishToMavenLocal --stacktrace
• python3 .github/scripts/validate_publish_plugin_publications.py
• ./gradlew :plugin_base:publishAllPublicationsToCentralStagingRepository -PcentralPublish=true --dry-run --stacktrace