inputs.opcua_listener: remote key size to large

### Relevant telegraf.conf

```toml
[[inputs.opcua_listener]]
name = "opcua_%plant%_%type%"
endpoint = "%server_url%"
connect_timeout = "10s"
request_timeout = "5s"
security_policy = "None"
security_mode = "None"
certificate = ""
private_key = ""
auth_method = "UserName"
username = "%username%"
password = "%password%"
timestamp = "gather"
timestamp_format = ""
nodes = [
"%nodes%"
]
```

### Logs from Telegraf

```text
2026-08-14T09:09:12Z I! Starting Telegraf 1.39.3 brought to you by InfluxData the makers of InfluxDB
2026-08-14T09:09:12Z I! Available plugins: 245 inputs, 9 aggregators, 35 processors, 26 parsers, 68 outputs, 8 secret stores
2026-08-14T09:09:12Z I! Loaded inputs: cpu disk diskio kernel mem opcua_listener processes swap system
2026-08-14T09:09:12Z I! Loaded aggregators:
2026-08-14T09:09:12Z I! Loaded processors:
2026-08-14T09:09:12Z I! Loaded secretstores:
2026-08-14T09:09:12Z I! Loaded outputs: influxdb
2026-08-14T09:09:12Z I! Tags enabled: host=Test
2026-08-14T09:09:12Z I! [agent] Config: Interval:10s, Quiet:false, Hostname:"Test", Flush Interval:10s
2026-08-14T09:09:12Z W! [agent] The default value of 'skip_processors_after_aggregators' will change to 'true' with Telegraf v1.40.0! If you need the current default behavior, please explicitly set the option to 'false'!
2026-08-14T09:09:13Z I! error encrypting user password: opcua: remote key size should be 128-256 bytes, got 384 bytes
2026-08-14T09:09:13Z E! [telegraf] Error running agent: starting input inputs.opcua_listener: endpoint "opc.tcp://xzy.xyz.xyz.xyz:4840": connection failed: opcua: remote key size should be 128-256 bytes, got 384 bytes
```

### System info

Docker version 20.10.5+dfsg1, build 55c4c88

### Docker

version: '3.6'
services:
telegraf:
image: telegraf:1.39.3
container_name: telegraf
restart: always
volumes:
- ./configs/:/etc/telegraf/telegraf.d/
- ./telegraf.env:/etc/default/telegraf
- "/etc/timezone:/etc/timezone:ro"
- "/etc/localtime:/etc/localtime:ro"
depends_on:
- influxdb
links:
- influxdb
ports:
- '8125:8125'
environment:
- TZ=Europe/Berlin

### Steps to reproduce

1. run telegraf config against a OPC-UA Server on a Siemens PLC with firmware > 4.x
2.
3.
...

### Expected behavior

remote keys of any size (e.g. larger than 256 bytes) should be accepted

### Actual behavior

key needs to be 128-256 bytes in length

### Additional info

login and subscribing to the OPC Server does work using UA-Expert without facing the issue