inputs.internet_speed: upload speed severely under-reported in v1.40.0

### Relevant telegraf.conf

```toml
# Test internet speed
[[inputs.internet_speed]]
interval = "1h"
collection_offset = "45m"
memory_saving_mode = false
test_mode = "multi"
```

### Logs from Telegraf

```text
No errors or warnings are logged, even with --debug. The plugin gathers and writes metrics successfully; only the value of the `upload` field is wrong.
```

### System info

Telegraf 1.40.0, Docker 29.8.0, Debian 13 Trixie

### Docker

_No response_

### Steps to reproduce

1. Run `[[inputs.internet_speed]]` on a host whose real upload is well above the reported value (e.g. a ~100+ Mbps connection).
2. Record the `upload` field on Telegraf **1.39.x**.
3. Upgrade to Telegraf **1.40.0** and record the `upload` field again for the same host/link.
4. Compare: `download` is unchanged and correct, but `upload` drops substantially.

Observed here: real upload >100 Mbps, reported upload ~28 Mbps; download reported ~118 Mbps and correct.

### Expected behavior

The `upload` field should reflect actual upload throughput, consistent with pre-**1.40.0** results and with browser/CLI speedtest runs from the same host.

### Actual behavior

The `upload` field is significantly under-reported. The `download` field is unaffected.

### Additional info

v**1.40.0** bumped github.com/showwin/speedtest-go from **1.7.10** to **1.8.2** (via #19425 and #19596). The upload measurement changed in speedtest-go PR showwin/speedtest-go#267 ("fix(upload): count only confirmed request bytes").

Start at the [[inputs.internet_speed]] entry point and inspect the github.com/showwin/speedtest-go upgrade from 1.7.10 to 1.8.2, especially the upload measurement change in PR #267. Reproduce on the same host with Telegraf 1.39.x and 1.40.0, then verify that upload no longer under-reports while download remains correct.

Written by the indexing model from the issue text.