Integrated Enterprise Explorer does not consume documented preconfigured connection config

@tomklapka is already working on this.

Since Sep 7, 2026.

## Summary

InfluxDB 3 Enterprise 3.11.2 can serve the integrated Explorer Web UI when querier pods start with `--mode=query,webui` and `--webui-session-secret`, but local testing showed that the integrated Web UI does not appear to consume the documented Explorer preconfigured connection values.

The same documented `config.json` preconfiguration worked when using the standalone Explorer container as a companion Kubernetes Deployment.

This is related to the Explorer follow-up from #817.

## Environment Tested

- InfluxDB 3 Enterprise: `3.11.2`
- Helm chart baseline: released `charts/influxdb3-enterprise` after #825, `appVersion: "3.11.2"`
- Local Kubernetes: Rancher Desktop / k3s on macOS arm64
- Deployment shape: five-node local Enterprise cluster
- 2 ingesters
- 2 queriers
- 1 compactor
- Integrated Explorer endpoint tested through querier Service on port `8181`
- Standalone Explorer companion tested with image `influxdata/influxdb3-ui:1.9.0` on port `8888`

## What Was Configured for Integrated Explorer

Querier pods were configured with:

```text
--mode=query,webui
--webui-session-secret=$(INFLUXDB3_WEBUI_SESSION_SECRET)
```

The documented Explorer preconfiguration was mounted and exposed using both forms:

```text
/app-root/config/config.json
DEFAULT_INFLUX_SERVER
DEFAULT_INFLUX_DATABASE
DEFAULT_API_TOKEN
DEFAULT_SERVER_NAME
```

The mounted `config.json` followed the documented shape:

```json
{
"DEFAULT_INFLUX_SERVER": "http://127.0.0.1:8181",
"DEFAULT_INFLUX_DATABASE": "explorer_demo",
"DEFAULT_API_TOKEN": "",
"DEFAULT_SERVER_NAME": "Local InfluxDB 3 Enterprise 3.11.2"
}
```

## Expected Behavior

For a fresh browser/session against the integrated Enterprise Explorer, the UI should load or seed the default server connection from `config.json` or the corresponding `DEFAULT_*` environment values.

After session creation, this endpoint should return the configured active server instead of `null`:

```bash
curl -sS -c cookies.txt -b cookies.txt http://localhost:8181/api/sessions
curl -sS -c cookies.txt -b cookies.txt http://localhost:8181/api/server-config/active
```

## Actual Behavior

Integrated Enterprise Explorer served successfully, but a fresh session did not have an active server configuration.

Observed signals:

```text
GET /api/server-config/active -> null
GET /api/server-config -> []
```

The UI therefore continued to show the first-run server configuration flow instead of opening with the local Enterprise cluster preconfigured.

## Control Test: Standalone Explorer Worked

Using the same Kubernetes namespace and the same target querier Service, a standalone Explorer companion Deployment did consume the documented preconfigured connection.

Standalone Explorer setup:

```text
image: influxdata/influxdb3-ui:1.9.0
args: --mode=admin
config: /app-root/config/config.json
server: http://-querier..svc.cluster.local:8181
```

For the standalone Explorer endpoint:

```bash
curl -sS -c cookies.txt -b cookies.txt http://localhost:8888/api/sessions
curl -sS -c cookies.txt -b cookies.txt http://localhost:8888/api/server-config/active
```

The active config returned the expected connection, and the UI showed:

- Product: InfluxDB 3 Enterprise
- Version: 3.11.2
- License: Trial
- Cluster/database metadata loaded from the Enterprise cluster

## Possible Root Cause

This looks like a behavior gap between standalone Explorer and integrated Enterprise Explorer.

Possibilities:

- the integrated Web UI WASM guest does not read `/app-root/config/config.json`
- the Enterprise host does not pass the documented `DEFAULT_*` values into the integrated Web UI/session bootstrap
- the integrated Enterprise Explorer has a different preconfiguration contract that is not currently documented

## Recommendations

1. Confirm whether integrated Enterprise Explorer is intended to support the same preconfiguration contract as standalone Explorer.
2. If yes, update the integrated Enterprise Web UI host/WASM path to seed the first server config from `config.json` or `DEFAULT_*` values during fresh session creation.
3. If no, update the Explorer docs to state that `/app-root/config/config.json` preconfiguration applies only to standalone Explorer, and document the supported integrated Web UI preconfiguration method.
4. Add a smoke test for integrated Enterprise Explorer:
- start `influxdb3 serve --mode=query,webui --webui-session-secret=...`
- provide `config.json` or `DEFAULT_*` values
- create a fresh UI session with `GET /api/sessions`
- assert `GET /api/server-config/active` returns the configured server
5. In the Helm chart, keep `webui.enabled` for the integrated UI, but use an optional standalone `explorer.enabled` companion Deployment for reliable Helm-managed preconfigured startup until the integrated behavior is confirmed or fixed.

## Documentation References

- Integrated Enterprise Explorer release notes: https://docs.influxdata.com/influxdb3/enterprise/release-notes/#enterprise-2
- Explorer preconfigured connection docs: https://docs.influxdata.com/influxdb3/explorer/install/#pre-configure-influxdb-connections
- Explorer mode docs: https://docs.influxdata.com/influxdb3/explorer/install/#choose-operational-mode