fix(ST-28): read deployed images for lockdown

[afaryy]

Summary

• read API and web container images from the deployed Cloud Run services during the guarded lockdown operation
• fail before Terraform applies if a required image or MCP endpoint is empty
• add a regression test preventing use of absent app-state image outputs

Verification

• npm test in infra/terraform/tools/config-validator — 8 passing
• git diff --check
• independent review completed with no blocking findings

Safety

This PR changes no live resources. The manual lockdown workflow remains the only path that can restrict Cloud Run ingress after merge.