Per-bundle status fields per OPA spec

[sspaink]

StatusPlugin.Status.buildStatusReport reports per-bundle {revision, active: true}. OPA Go's status payload has a richer shape under bundles.<name>: last_successful_activation, last_successful_download, last_request, code, message, metrics, plus the current revision. Without those, ops tooling that targets OPA can't consume the Java SDK's status without translation.

• Surface the per-bundle activation and download timestamps from BundlePlugin (depends on bundles Per-bundle activation status events #68).
• Include code and message for the most recent error if the bundle is in a failure state.
• Match OPA Go's field names exactly so existing dashboards/integrations work unchanged.
• Tests covering: healthy bundle, transient download failure, activation failure (interaction with bundles Keep last-good bundle on activation failure #66 last-good rollback).