minio S3 fixture charm fails install on Juju 4.x (pod-spec-set removed), blocking kyuubi-k8s and other S3-dependent test plans

## Summary

The `minio` charm used as the S3 backend fixture (`s3-integrator-minio`) in S3-dependent integration test bundles is a **legacy Kubernetes _podspec_ charm**. Its `install` hook calls the `pod-spec-set` hook tool, which **no longer exists in Juju 4.x**. The hook therefore fails deterministically, the S3 backend never comes up, and the artefact under test times out waiting for the model to settle.

This is **not** a bug in the charm under test — it is collateral damage from an incompatible test fixture. A rerun will fail identically, so these failures should not be retried or filed against the target charms.

## Root cause (verified in juju debug-log)

```
self._backend.pod_spec_set(spec, k8s_resources)
File ".../venv/ops/model.py", line 3472, in pod_spec_set
self._run('pod-spec-set', *args)
RuntimeError: command not found: pod-spec-set
```

- Environment: `juju:4/stable` (**4.0.14**), `ubuntu:22.04`, kubernetes, amd64
- Fixture: `s3-integrator-minio` = **minio rev 583** (`ckf-1.10/stable`)
- Juju 4.x removed support for podspec charms, so `pod-spec-set` is gone. The log confirms every sidecar charm reports *"not a Kubernetes podspec charm"* — only `minio` is podspec, and only `minio` dies.

## Causal chain

1. `minio` `install` hook → `pod-spec-set` → `RuntimeError: command not found` → `hook failed: "install"`.
2. `s3-integrator` stays `blocked (Missing parameters)` — waiting on minio.
3. `spark-integration-hub-k8s` → `blocked (Invalid S3 credentials)`.
4. `kyuubi-k8s` (target) → `blocked (Missing Object Storage backend)`.
5. `test_deploy` → `juju.backend.JujuWaitTimeoutError` waiting for `s3-integrator-minio`.

## Example failure

- Artefact under test: **kyuubi-k8s** rev 183 (track 3.5)
- Test case: `test_deploy`
- Test plan: `kyuubi-k8s:grafana-dashboard/grafana_dashboard/grafana-k8s:grafana-dashboard`
- CI run: https://github.com/canonical/charm-integration-testing/actions/runs/32655002132
- Test Observer: https://test-observer.canonical.com/#/charms/409789?testExecutionId=681956&testResultId=13058448

## Suggested fix

Replace the legacy podspec `minio` fixture with a Juju-4-compatible (sidecar) S3/MinIO backend, or pin S3-dependent test plans to Juju 3.x until a compatible fixture is available. Any charm whose test bundle deploys this `minio` fixture will fail on Juju 4.x until this is addressed.

---
_Filed by an AI triage assistant (GitHub Copilot) on behalf of @LucasPluta. Root cause verified against the juju debug-log for test execution 681956; please validate before acting._

Start by inspecting the S3-dependent test plans, the s3-integrator-minio fixture setup, and the test_deploy path that waits for model settlement; reproduce the failure on Juju 4.x and review the debug-log evidence. Done means the plans use a Juju-4-compatible backend or are pinned to Juju 3.x, with the affected integration tests no longer blocked by the minio install hook.

Written by the indexing model from the issue text.