[ci-scan] Test failure: System.Diagnostics.Tests.StackTraceTests.ToString_Async

> [!CAUTION]
> agentic threat detected
> Threat detection flagged this output in warn mode. Manual review is REQUIRED before any follow-up automation.
>
>
>
> Details
>
> The threat detection results could not be parsed.
>
> Review the [workflow run logs](https://github.com/dotnet/runtime/actions/runs/29791943554) for details.
>

## Build Information
Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1514774
Build error leg or test failing: runtime-libraries-interpreter (def 330) — linux-x64 / osx-arm64 Release — System.Diagnostics.StackTrace.Tests: System.Diagnostics.Tests.StackTraceTests.ToString_Async

## Error Details

Under the interpreter, async stack-trace frames are emitted with incorrect source line numbers (`:line 1` / `:line 3` instead of the real source lines), so `ToString_Async` cannot match its expected pattern requiring `...ThrowsSoonValueTaskSource...:line 4`. The failure reproduces in source build 1514774 and its follow-up build 1517739. This is distinct in scope from the mobile stack-trace tracker dotnet/runtime#124087 (Android / iOS CoreCLR); this signature is the interpreter configuration and no existing Known Build Error was found for it.

```
System.Diagnostics.Tests.StackTraceTests.ToString_Async(...) [FAIL]
Could not find expected pattern 'V2Methods\.ThrowsSoonValueTaskSource\(\).*ThrowsSoonValueTaskSource.*\.cs:line 4' in exception text:
System.Exception: Exception from ThrowsSoonValueTaskSourceImpl
at System.Diagnostics.V2Methods.ThrowsSoonValueTaskSourceImpl.GetResult(Int16 token) in /_/src/libraries/System.Diagnostics.StackTrace/tests/ThrowsSoonValueTaskSourceImpl.cs:line 1
at System.Diagnostics.V2Methods.ThrowsSoonValueTaskSource() in /_/src/libraries/System.Diagnostics.StackTrace/tests/ThrowsSoonValueTaskSource.cs:line 3
at System.Diagnostics.Tests.StackTraceTests.ToString_Async(...) in /_/src/libraries/System.Diagnostics.StackTrace/tests/StackTraceTests.cs:line 766 starting at index 246.
```

## Error Message

```json
{
"ErrorMessage": [
"System.Diagnostics.Tests.StackTraceTests.ToString_Async",
"Could not find expected pattern 'V2Methods\\.ThrowsSoonValueTaskSource"
],
"ErrorPattern": "",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

---
Filed by `ci-failure-scan` (detection only). `ci-failure-fix` walks open `[ci-scan]` KBEs and either opens a small fix PR or comments to loop in owners — it never disables the test.

> [!NOTE]
> This issue was generated by the automated `ci-failure-scan` GitHub Copilot workflow.

> Generated by [CI Outer-Loop Failure Scanner](https://github.com/dotnet/runtime/actions/runs/29791943554) · 537.2 AIC · ⊞ 19.5K · [◷](https://github.com/search?q=repo%3Adotnet%2Fruntime+is%3Aissue+%22gh-aw-workflow-call-id%3A+dotnet%2Fruntime%2Fci-failure-scan%22&type=issues)

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

Start with System.Diagnostics.StackTrace/tests/StackTraceTests.cs and the referenced ThrowsSoonValueTaskSource.cs and ThrowsSoonValueTaskSourceImpl.cs files; run ToString_Async in the runtime-libraries-interpreter configuration to reproduce the line-number mismatch. Trace how interpreter async frames produce source locations, then verify the test reports the expected lines 4 and 3 in the exception text.

Written by the indexing model from the issue text.