ci: require env-scoped longport app secrets

[Pigbibi]

Summary

• require LONGPORT_APP_KEY_SECRET_NAME and LONGPORT_APP_SECRET_SECRET_NAME for each GitHub Environment in sync-cloud-run-env
• remove GitHub secret fallback for LongPort app key/app secret so HK/SG do not silently inherit shared repo defaults
• update README and workflow test to match the env-scoped Secret Manager setup

Verification

• bash tests/test_sync_cloud_run_env_workflow.sh
• python -m unittest discover -s tests -v
• ruff check .

Operational follow-up

• removed repo-level variables LONGPORT_APP_KEY_SECRET_NAME and LONGPORT_APP_SECRET_SECRET_NAME
• confirmed longbridge-hk and longbridge-sg each have their own env-scoped secret-name vars
• granted roles/secretmanager.secretAccessor on longport-app-key-hk and longport-app-secret-hk to longbridge-platform-runtime@longbridgequant.iam.gserviceaccount.com