test_deploy fails for indico — smtp-integrator blocked on invalid configuration: host

**Note: This issue was generated with AI assistance (GitHub Copilot) based on automated log analysis and triage.**
Filed by @canonical/solutions-qa

---

## Summary

`test_deploy` consistently fails for `indico` rev 334 (`latest/edge`) on the `smtp-legacy` test plan. The neighbor charm `smtp-integrator` rev 121 deploys into `blocked` state immediately because no `host` configuration is provided in the generated bundle. The 15-minute idle wait then times out waiting for `neighbor/0` to become active.

## Error

```
juju.backend.JujuWaitTimeoutError: Timed out while waiting for applications: [active], units: [active] (applications: ['neighbor'], units: ['neighbor/0'])
```

Juju status at timeout:

```
App Version Status Scale Charm Channel Rev Message
neighbor blocked 1 smtp-integrator latest/stable 121 invalid configuration: host

Unit Workload Agent Machine Message
neighbor/0* blocked idle 0 invalid configuration: host
```

## Details

- **Charm under test**: `indico` rev 334, channel `latest/edge`
- **Neighbor charm**: `smtp-integrator` rev 121, channel `latest/stable`
- **Test plan**: `integration/indico:smtp-legacy/smtp/smtp-integrator:smtp-legacy` (CMR, openstack neighbor cloud)
- **Environment**: `juju:3/stable ubuntu:20.04 cmr/kubernetes/openstack`
- **Juju version**: 3.6.23

## Root cause

`smtp-integrator` rev 121 requires the `host` configuration option to be set before it can become active. The generated test bundle deploys `smtp-integrator` without this option, causing it to block immediately.

## Steps to reproduce

Run integration tests for `indico` rev 334 on the `smtp-legacy` test plan in a `cmr/kubernetes/openstack` environment. The failure is 100% reproducible — all 5 consecutive executions on this artefact fail at `test_deploy` for the same reason (executions 520599, 520602, 520604, 520606, 520608).

## Suggested fix

Add a charm override for `smtp-integrator` that provides a dummy/placeholder value for the `host` configuration option, similar to the existing override for `superset-k8s` (#496).

## Test Observer

https://test-observer.canonical.com/#/charms/407775?testExecutionId=520608&testResultId=11217709

Start with the generated bundle and the existing charm override referenced in issue #496. Run the indico smtp-legacy integration test or inspect its deployment configuration to confirm that smtp-integrator lacks host. Done means the neighbor becomes active and test_deploy no longer times out.

Written by the indexing model from the issue text.