Support ADC discovery for GCP OIDC auth

[tonywok]

Summary

• Add a regression test for google-cloud/auth-oidc that exercises Application Default Credentials from the task command process.
• Verify ADC is available through Google's well-known credentials file at $HOME/.config/gcloud/application_default_credentials.json.
• Write the generated external account credentials JSON to that well-known ADC file during the before hook so Terraform/Google SDK clients can discover credentials without relying on same-task env propagation.
• Remove the ADC file in the after hook while preserving the existing GOOGLE_APPLICATION_CREDENTIALS output for dependent tasks.

Testing

• Observed the new ADC regression test fail before the implementation change.
• Validated YAML locally.
• Validated the generated before hook with bash -n.