[macOS 26] dotnet dev-certs fails to save HTTPS developer certificate in Helix

## Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1586491
Leg Name: `browser-wasm osx Release LibraryTests_SmokeMac - osx.26.arm64.open`

Also observed in https://dev.azure.com/dnceng-public/public/_build/results?buildId=1584957, where the affected work item passed after Helix resubmission.

## Error Details

The WASM test work item fails during environment setup, before tests execute:

```text
+ dotnet dev-certs https
There was an error saving the HTTPS developer certificate to the current user personal certificate store.
+ export _commandExitCode=2
```

Confirmed occurrences:

- Build 1586491: `WasmTestOnChrome-MONO-ST-System.Runtime.Intrinsics.Tests` on `dci-mac-build-480`, macOS 26.4.1
- Build 1584957: `WasmTestOnChrome-MONO-ST-System.Runtime.InteropServices.JavaScript.Tests` on `dci-mac-build-462`, macOS 26.4; the resubmitted work item passed

The subsequent missing `xharness-event-processor.py` message is secondary to the setup command exiting with code 2.

## Error Message

```json
{
"ErrorMessage": "There was an error saving the HTTPS developer certificate to the current user personal certificate store.",
"BuildRetry": true,
"ExcludeConsoleLog": false
}
```

> [!NOTE]
> This issue was drafted and filed with GitHub Copilot.

Start with the Helix environment setup command `dotnet dev-certs https` and the failed Build 1586491 work item on macOS 26.4.1. Compare it with the resubmitted Build 1584957 work item, where the command succeeded, and determine what setup change is needed so the certificate is saved and the WASM smoke tests can proceed.

Written by the indexing model from the issue text.