[wasm] foreground-shutdown test fails with PlatformNotSupportedException from Thread.Start

### Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1540634
Build error leg or test failing: `baseservices/threading/regressions/2164/foreground-shutdown/foreground-shutdown.cmd`
Pull request: N/A

### Error Message

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Projects/Build%20Analysis/KnownIssues.md#how-to-fill-out-a-known-issue-error-message-section).

```json
{
"ErrorMessage": [
"Unhandled exception. System.PlatformNotSupportedException: Operation is not supported on this platform.",
"System.Runtime.CompilerServices.RuntimeFeature.ThrowIfMultithreadingIsNotSupported()"
],
"ErrorPattern": "",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

### Suggested fix
Exclude the test for `browser`/single-threaded wasm, or gate it on `FeatureWasmManagedThreads`.

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1540634
**Error message validated:** `[Unhandled exception. System.PlatformNotSupportedException: Operation is not supported on this platform. System.Runtime.CompilerServices.RuntimeFeature.ThrowIfMultithreadingIsNotSupported()`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 8/5/2026 7:36:55 PM UTC

### Report
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|0|0|