Improve RebuildableSetCache structured logging

What does this MR do and why?

Improve RebuildableSetCache structured logging

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/586463

Problem

During the rollout of the ref_cache_with_rebuild_queue feature flag, three logging issues were identified:

1. json.meta.project was missing in logs when accessed from GraphQL code paths where the project was not in the ApplicationContext.
2. json.ref displayed as NaN in Kibana due to a field name collision with an existing indexed field.
3. json.message was always RebuildableSetCache, duplicating json.class without adding value. Additionally, high-volume info logs (e.g. cache_hit on every page load) could significantly increase log volume at scale.

Solution

• Wrap log emission in Gitlab::ApplicationContext.with_context(project:) so json.meta.project is always populated.
• Nest domain-specific fields under a rebuildable_cache key (e.g. json.rebuildable_cache.ref) to avoid Kibana field collisions.
• Set json.message to the event name (e.g. cache_hit, rebuild_started) for better readability.
• Gate all non-error info logs behind a new ref_cache_verbose_logging feature flag (disabled by default). Error logs always emit.

References

• Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/586463
• Epic: https://gitlab.com/groups/gitlab-org/-/epics/20516

Screenshots or screen recordings

{
  "severity": "INFO",
  "time": "2026-05-13T09:25:57.948Z",
  "correlation_id": "e102b8ed3b9962f595315c6a179bfd13",
  "meta.project": "namespace1/project-1",
  "meta.root_namespace": "namespace1",
  "message": "rebuild_started",
  "class": "Gitlab::Repositories::RebuildableSetCache",
  "rebuildable_cache": {
    "event": "rebuild_started",
    "cache_key": "branch_names",
    "canonical_count": 4
  }
}

How to set up and validate locally

1. Enable the feature flags:

   Feature.enable(:ref_cache_with_rebuild_queue)
   Feature.enable(:ref_cache_verbose_logging)

2. Push a branch to a project.
3. Check log/application_json.log for entries with "message": "simple_update" or "message": "cache_hit".
4. Verify rebuildable_cache nested key contains event, cache_key, ref, etc.
5. Verify meta.project is populated.
6. Disable verbose logging:

   Feature.disable(:ref_cache_verbose_logging)

7. Repeat push — verify no info-level RebuildableSetCache logs appear.

MR acceptance checklist

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