feat: add OCI Helm registry support for agent deployments

[smoreinis]

Summary

This PR adds support for deploying agents using OCI-based Helm registries (such as Google Artifact Registry) as an alternative to classic Helm repositories.

Key Changes

• New environment config fields: Added helm_oci_registry and helm_chart_version to AgentEnvironmentConfig for configuring OCI-based deployments
• Auto-authentication: Implemented automatic login to Google Artifact Registry using gcloud auth print-access-token
• Latest chart version fetching: Added --use-latest-chart CLI flag that queries GAR for the most recent chart tag
• Dual-mode support: The deploy command now intelligently switches between classic Helm repo mode and OCI registry mode based on environment configuration

Usage

OCI mode (new) - Set helm_oci_registry in your environment config:

environments:
  production:
    helm_oci_registry: "us-west1-docker.pkg.dev/my-project/helm-charts"
    helm_chart_version: "0.2.0"  # optional, defaults to CLI default

Fetch latest chart version:

agentex agents deploy --manifest manifest.yaml --use-latest-chart

Classic mode (unchanged) - Continue using helm_repository_name and helm_repository_url as before.

Test plan

• Test deployment with OCI registry configuration
• Test --use-latest-chart flag fetches correct version from GAR
• Verify classic Helm repo mode still works unchanged
• Test error handling when gcloud is not installed or authenticated