feat: show valid/total relations count in status after haproxy-route reconciliation

[Copilot]

What this PR does

After haproxy-route reconciliation, displays a unified count of valid vs total relations (combining both haproxy-route and haproxy-route-tcp) in the unit's ActiveStatus message. Example output:

active: 2/3 valid relations

When no relations exist, the status message is empty.

Changes:

• _configure_haproxy_route now returns a status message string
• Added _get_haproxy_route_status_message helper that computes combined valid/total across both haproxy-route and haproxy-route-tcp using relation_ids_with_invalid_data and relation_ids_with_invalid_data_tcp
• _reconcile passes the message to ActiveStatus

Why we need it

Operators currently have no visibility into which haproxy-route/haproxy-route-tcp relations are valid after config reconciliation. This makes debugging misconfigured requirers harder than necessary.

Test plan

• Updated existing unit tests (test_spoe_auth, test_two_spoe_auth) to assert the new status message format
• Full unit test suite passes

Review focus

• Status message format: "x/y valid relations" — single combined count for both interfaces
• Edge case: when no relations exist at all, the message is empty

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 Terraform: terraform fmt passes and tflint reports no errors