Fix content-cache-k8s hanging in maintenance when nginx-proxy is consumed via CMR

[Copilot]

In a Cross-Model Relation (CMR), Juju represents remote units with proxy names like remote-<uuid>/0 instead of real application unit names. The charm was using these tokens to build headless pod DNS entries (remote-<uuid>-0.<svc>-endpoints.<local-model>.svc.cluster.local) that don't exist, causing nginx to fail on startup and leaving the unit stuck in maintenance: Updating pebble layer config.

What this PR does

• Detects CMR proxy units in _make_env_config by checking for the remote- prefix on unit names
• When CMR is detected, constructs a service-level K8s DNS entry instead of a pod-level headless one:

  http://{service-name}.{service-namespace}.svc.cluster.local:{service-port}
  

  service-namespace comes from the relation app data (automatically populated by NginxRouteRequirer with the requirer's model name); falls back to the local model name if absent
• Preserves existing pod-level headless DNS behaviour for same-model relations

Why we need it

Cross-model relations are a standard Juju capability. CMR deployments were completely broken — the charm would never reach active and the hook would fail on every retry.

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration)
• If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• If this PR involves a Grafana dashboard: I added a screenshot of the dashboard
• If this PR involves Rockcraft: I updated the version

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Test plan

Two new unit tests added:

• test_make_env_config_with_cmr_proxy_relation — verifies service-level DNS with service-namespace present
• test_make_env_config_with_cmr_proxy_relation_no_namespace — verifies fallback to local model name when service-namespace is absent

Review focus

The CMR detection heuristic (unit.name.startswith("remote-")) matches Juju's documented naming convention for CMR proxy units. No Juju API exists to query this directly, so the prefix check is the standard approach.

[Copilot]

Pull request overview

Fixes content-cache-k8s getting stuck in maintenance: Updating pebble layer config when the nginx-proxy relation is established via Juju Cross-Model Relations (CMR), where remote units are represented as remote-<uuid>/<n> and cannot be used to construct pod-level headless DNS names.

Changes:

• Update _make_env_config to detect CMR proxy unit names (remote-...) and, when detected, build a service-level Kubernetes DNS backend URL using service-name, service-namespace (fallback to local model name), and service-port.
• Preserve existing same-model behavior that builds pod-level headless DNS entries from unit names.
• Add unit tests validating the service-level DNS behavior for CMR (with and without service-namespace present).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/charm.py	Adds CMR detection and switches backend URL construction to service-level DNS for CMR relations.
tests/unit/test_charm.py	Adds unit tests covering CMR proxy relations and the service-namespace fallback behavior.

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[minulo]

lgtm