[maccatalyst] `System.Diagnostics.TraceSource.Config.Tests` fails with `ClientConfigurationHost` `MissingMethodException`

**Related runtime issues**: https://github.com/dotnet/runtime/issues/126460, https://github.com/dotnet/runtime/issues/74244, https://github.com/dotnet/runtime/issues/49062
**Suggested labels**: `Known Build Error`, `test-failure`, `os-maccatalyst`, `arch-x64`, `arch-arm64`, `area-System.Diagnostics.TraceSource`, `area-System.Configuration`, `untriaged`

## Build Information

Build: [20260404.2](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1367026), [20260405.1](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1367268), [20260406.2](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1368287)
Build error leg or test failing: `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.ConfigWithEvents_RuntimeListener`, `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.Refresh_ChangeSwitch`, `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.Refresh_RemoveSource`, `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.RuntimeFilterChange`, `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.UnsupportedAttribute_Throws`, `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.AllTypes`, `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.Refresh_RemoveSwitch`, `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.Switch_MissingValue_Throws`, `System.Diagnostics.TraceSource.Config.Tests.WorkItemExecution`
Affected CI jobs: `build_maccatalyst_x64_Release_AllSubsets_CoreCLR`, `build_maccatalyst_arm64_Release_AllSubsets_CoreCLR`

## Error Message

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

```json
{
"ErrorMessage": "System.MissingMethodException : Cannot dynamically create an instance of type 'System.Configuration.ClientConfigurationHost'. Reason: No parameterless constructor defined.",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

## Summary

`System.Diagnostics.TraceSource.Config.Tests` is failing consistently on MacCatalyst CoreCLR Release after the full MacCatalyst CoreCLR library legs were enabled in [dotnet/runtime#125441](https://github.com/dotnet/runtime/pull/125441). The generic XHarness bucket text mentions `Process log exited with 137`, but the uploaded `testResults.xml` and app log show the real failure is deterministic and test-specific:

```text
System.MissingMethodException : Cannot dynamically create an instance of type 'System.Configuration.ClientConfigurationHost'. Reason: No parameterless constructor defined.
at System.Configuration.ConfigurationManager.OpenExeConfigurationImpl(...)
at System.Diagnostics.TraceSourceConfigTests.ConfigurationTests.CreateAndLoadConfigFile(String filename)
```

Observed behavior:

- All 8 tests in `System.Diagnostics.TraceSourceConfigTests.ConfigurationTests` fail in ~1 second on both `maccatalyst-x64` and `maccatalyst-arm64`.
- Every failing test goes through `CreateAndLoadConfigFile()` in `src/libraries/System.Diagnostics.TraceSource/tests/System.Diagnostics.TraceSource.Config.Tests/TraceSourceWithConfigurationTests.cs`.
- The Apple test app is built through `ProxyProjectForAOTOnHelix.proj` with `TrimMode = link` (confirmed from the uploaded `AOTBuild.binlog`), which matches the known trim incompatibility of `System.Configuration.ConfigurationManager` described in [#49062](https://github.com/dotnet/runtime/issues/49062).
- These tests are already excluded on `tvOS`, `iOS`, and `Android` via [#74244](https://github.com/dotnet/runtime/issues/74244), but `MacCatalyst` is not currently included in that `ActiveIssue`.

### Mono comparison

I checked the **actual uploaded xUnit `testResults.xml`** for the Mono MacCatalyst work items in build [20260406.2](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1368287), not just the parent job status:

- `maccatalyst-arm64 Release AllSubsets_Mono` → Helix job `ee05f111-cc24-47d1-8366-98c250455730`
- `maccatalyst-x64 Release AllSubsets_Mono` → Helix job `ed8d84e6-59a6-46f2-8a43-2f400f24c10a`

For **both** Mono runs, `System.Diagnostics.TraceSource.Config.Tests` reports:

- `total="8" passed="8" failed="0" skipped="0"`
- all 8 `ConfigurationTests.*` cases executed and passed

I also spot-checked the previous build [20260405.1](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1367268); the same work item passed there on both Mono MacCatalyst queues as well. So this is **not** a case where Mono is silently skipping the test assembly: the tests genuinely run and pass on Mono, and the problem appears specific to the CoreCLR MacCatalyst configuration.

### Likely causative commit

The most likely commit that **introduced this failure into CI** is:

- `0a726991ba` — [dotnet/runtime#125441](https://github.com/dotnet/runtime/pull/125441), **"Reduce Apple mobile CI matrix"** (merged 2026-04-02)

Why this is the strongest candidate:

- The PR changed `eng/pipelines/extra-platforms/runtime-extra-platforms-maccatalyst.yml` to add the new full **`maccatalyst-{x64,arm64} Release AllSubsets_CoreCLR` library-test legs**.
- Those new CoreCLR library jobs build test apps with:
- `/p:UseMonoRuntime=false`
- `/p:UseNativeAOTRuntime=false`
- `/p:UsePortableRuntimePack=false`
- `/p:EnableAggressiveTrimming=true`
- In builds **before** that change (for example [20260401.5 / build 1363261](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1363261) and [20260331.6 / build 1361338](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1361338)), the MacCatalyst CoreCLR coverage was limited to **`maccatalyst-arm64 Release AllSubsets_CoreCLR_Smoke`** plus runtime-test legs; the new full library-test job where `System.Diagnostics.TraceSource.Config.Tests` now fails did not exist yet.

I do **not** see evidence of a recent behavior-changing product commit in the relevant libraries:

- `TraceSourceWithConfigurationTests.cs` was last changed substantively in older PRs such as [#73087](https://github.com/dotnet/runtime/pull/73087) and Android/mobile test annotation updates, not in the April 2026 regression window.
- `ClientConfigurationHost.cs` has no recent behavioral change; the only 2026 entry in file history is a mechanical volatile cleanup.

So the best read is:

1. **Exposure commit**: `0a726991ba` / `#125441` made these tests run in a new MacCatalyst CoreCLR + aggressive trimming configuration.
2. **Underlying product issue**: older `System.Configuration.ConfigurationManager` trim incompatibility already known from [#49062](https://github.com/dotnet/runtime/issues/49062).

This looks like a newly exposed mobile/trim incompatibility rather than a fresh product regression in `System.Diagnostics.TraceSource`. A practical next step is to extend the existing mobile skip for `System.Diagnostics.TraceSource.Config.Tests` to include `MacCatalyst`, unless there is interest in making this `ConfigurationManager` scenario work in trimmed Apple test apps.

> [!NOTE]
> This issue was filed with the assistance of GitHub Copilot.

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