Re-check whether variable filters can stay on native PROMQL

## Summary

Panels whose PromQL uses a Grafana template variable in a label matcher (e.g. `{device=\"$device_filtered\"}`) are routed to **native ES|QL** instead of **native PROMQL**, even with `--translation-mode native`.

That fallthrough is intentional today (issue #230 / PR #236): params inside an opaque `PROMQL …` command were observed to stay unbound in Kibana (`Parameter [?var] value not found`), so migration surfaces the filter as `WHERE … ?var` instead.

But the platform stack that #64 assumed is now in place:

| Layer | Ticket | Claimed outcome |
| --- | --- | --- |
| Elasticsearch | [elastic/elasticsearch#148620](https://github.com/elastic/elasticsearch/issues/148620) / [PR#149744](https://github.com/elastic/elasticsearch/pull/149744) | `?params` in PromQL label matchers |
| Kibana | [elastic/kibana#271215](https://github.com/elastic/kibana/issues/271215) / [PR#271244](https://github.com/elastic/kibana/pull/271244) | Variable controls bind into PromQL label matchers |
| Migration | #64 / PR #133 | `$var` → `?var` (and `=` → `=~` for multi/All), keep native PROMQL |

**Ask:** re-verify on current Kibana/ES (e.g. 9.5) whether a Lens **dashboard** panel with native PROMQL + a matching control actually binds. If yes, #230's guard is stale and we should keep these panels on native PROMQL (per #64). If no, document the remaining Kibana gap and keep the fallthrough.

Related: #316 already notes the PROMQL skip as background for the ES|QL `TBUCKET` bug; this issue is specifically about **whether that skip is still correct**.

## Expected vs actual (Issue 1)

Grafana source:

```promql
node_disk_read_bytes_total{device="$device_filtered"}
```

(`device_filtered` is multi-select.)

**Expected (native PROMQL, #64 + dashboard timing from #318):**

```esql
PROMQL index=metrics-* value=(node_disk_read_bytes_total{device=~?device_filtered})
```

**Actual today:** native PROMQL skipped → ES|QL TS path (e.g. `TBUCKET(…)` + `WHERE` / params for the device control).

Code guard: `panels.py` returns `None` from the native PROMQL path when `_promql_label_matcher_has_template_variable(expr)` (comment cites #230).

## Reproduction

1. Migrate a Grafana panel with `{label=\"$var\"}` (or `=~` / multi) under native PromQL translation.
2. Confirm the uploaded panel is ES|QL, not `PROMQL … value=(…{label=~?var})`.
3. Manually paste the expected PROMQL above into a Lens dashboard panel with a `device_filtered` control on Kibana 9.5+ and record whether the control binds (screenshot).

```bash
obs-migrate migrate \
--source grafana \
--assets dashboards \
--input-mode files \
--input-dir /path/to/folder/with/dashboard.json \
--kibana-url \"$KIBANA_URL\" \
--kibana-api-key \"$KIBANA_API_KEY\" \
--es-url \"$ES_URL\" \
--es-api-key \"$ES_API_KEY\" \
--upload
```

Minimal source (Issue 1 panel + vars)

```json
{
"title": "Issue 1",
"uid": "afst21a0rak8wf",
"panel": {
"title": "Panel Title",
"type": "timeseries",
"targets": [
{
"datasource": {
"type": "prometheus",
"uid": "prometheus"
},
"disableTextWrap": false,
"editorMode": "builder",
"expr": "node_disk_read_bytes_total{device=\"$device_filtered\"}",
"fullMetaSearch": false,
"includeNullMetadata": true,
"legendFormat": "__auto",
"range": true,
"refId": "A",
"useBackend": false
}
],
"interval": null
},
"templating": [
{
"name": "device_all",
"type": "query",
"multi": true,
"includeAll": null,
"query": {
"qryType": 1,
"query": "label_values(device)",
"refId": "PrometheusVariableQueryEditor-VariableQuery"
}
},
{
"name": "device_filtered",
"type": "query",
"multi": true,
"includeAll": null,
"query": {
"qryType": 1,
"query": "label_values(node_disk_read_bytes_total{device!=\"nbd1\"},device)",
"refId": "PrometheusVariableQueryEditor-VariableQuery"
}
}
]
}
```

Start in panels.py at the native PROMQL guard using _promql_label_matcher_has_template_variable, then run the provided obs-migrate command with the minimal Grafana dashboard. Manually test the expected PROMQL in a Lens dashboard on Kibana 9.5+ with the device_filtered control. Done means recording whether the control binds and either removing the stale fallthrough or documenting the remaining Kibana gap.

Written by the indexing model from the issue text.