[Mobile build] CI failure on 8.0: MSBuild error in <XHarnessAppBundleToTest> because of a missing directory

## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1596752&view=logs&j=1a7f2d0f-403f-5f1a-afbb-20466d7ac54e
Build error leg or test failing: Build / ios-arm64 Release AllSubsets_NativeAOT / Send to Helix
Pull request: N/A

## Error Message

**DO NOT USE JSON BELOW IF THIS IS A BUILD BREAK** otherwise build analysis will allow pull requests to merge that break the build worse. For a build break, do not use this issue form. Make a regular new issue.

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Build%20Analysis/KnownIssueJsonStepByStep.md).

```json
{
"ErrorPattern": "sendtohelix-mobile\\.targets\\(\\d+,\\d+\\): error MSB4184: The expression \"\\[System\\.IO\\.Directory\\]::GetDirectories\\(.*?artifacts/helix/tests/?, \\*\\.app, System\\.IO\\.SearchOption\\.AllDirectories\\)\" cannot be evaluated\\. Could not find a part of the path '.*?artifacts/helix/tests'",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

```
src/libraries/sendtohelix-mobile.targets(87,9): error MSB4184: The expression "[System.IO.Directory]::GetDirectories(/Users/runner/work/1/s/artifacts/helix/tests/, *.app, System.IO.SearchOption.AllDirectories)" cannot be evaluated. Could not find a part of the path '/Users/runner/work/1/s/artifacts/helix/tests'.
```

First reported here: https://github.com/dotnet/runtime/issues/130860
Fix seems to be incomplete and still failing on the servicing release/8.0-staging branch

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1596752
**Error message validated:** `[sendtohelix-mobile\.targets\(\d+,\d+\): error MSB4184: The expression "\[System\.IO\.Directory\]::GetDirectories\(.*?artifacts/helix/tests/?, \*\.app, System\.IO\.SearchOption\.AllDirectories\)" cannot be evaluated\. Could not find a part of the path '.*?artifacts/helix/tests'`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 9/15/2026 2:27:05 PM UTC

### Report
|Build|Repository|Step Name|Console log|Pull Request|
|---|---|---|---|---|
|[1596752](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1596752)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1596752/logs/613)||
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|1|1|1|

Start with src/libraries/sendtohelix-mobile.targets at line 87 and inspect the Send to Helix build logs linked in the issue. Compare the failing 8.0-staging or servicing-release behavior with the fix referenced from issue 130860. Done means the missing artifacts/helix/tests directory no longer causes MSB4184 and the ios-arm64 Release build completes its Send to Helix step.

Written by the indexing model from the issue text.