CI failure on 8.0: Failed to install xharness because of missing DotnetToolSettings.xml

## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1511237&view=logs&j=f29299c2-76db-5d02-ce4e-182614f08c53
Build error leg or test failing: Build / linux_bionic-arm64 Release AllSubsets_Mono / 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
{
"ErrorMessage": ["The settings file in the tool's NuGet package is invalid: Settings file 'DotnetToolSettings.xml' was not found in the package.", "Tool 'microsoft.dotnet.xharness.cli' failed to install."],
"BuildRetry": true,
"ExcludeConsoleLog": true
}
```

```
Skipping NuGet package signature verification.
The settings file in the tool's NuGet package is invalid: Settings file 'DotnetToolSettings.xml' was not found in the package.
Tool 'microsoft.dotnet.xharness.cli' failed to install. Contact the tool author for assistance.
/__w/1/s/.packages/microsoft.dotnet.helix.sdk/8.0.0-beta.26278.3/tools/xharness-runner/XHarnessRunner.targets(29,5): error : Failed to install the dotnet tool. Installation exited with 1. [/__w/1/s/src/libraries/sendtohelixhelp.proj]
/__w/1/s/.packages/microsoft.dotnet.helix.sdk/8.0.0-beta.26278.3/tools/xharness-runner/XHarnessRunner.targets(29,5): error : [/__w/1/s/src/libraries/sendtohelixhelp.proj]
##[error].packages/microsoft.dotnet.helix.sdk/8.0.0-beta.26278.3/tools/xharness-runner/XHarnessRunner.targets(29,5): error : Failed to install the dotnet tool. Installation exited with 1.

Build FAILED.
```

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

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1511237
**Error message validated:** `[The settings file in the tool's NuGet package is invalid: Settings file 'DotnetToolSettings.xml' was not found in the package. Tool 'microsoft.dotnet.xharness.cli' failed to install.`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 7/16/2026 5:14:37 PM UTC

### Report
|Build|Repository|Step Name|Console log|Pull Request|
|---|---|---|---|---|
|[1514458](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1514458)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1514458/logs/430)||
|[1511237](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1511237)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1511237/logs/309)||
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|1|2|2|

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1511237
**Error message validated:** `[The settings file in the tool's NuGet package is invalid: Settings file 'DotnetToolSettings.xml' was not found in the package. Tool 'microsoft.dotnet.xharness.cli' failed to install.`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 7/16/2026 5:14:37 PM UTC

### Report
|Build|Repository|Step Name|Console log|Pull Request|
|---|---|---|---|---|
|[1592053](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592053)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1592053/logs/1387)||
|[1591372](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591372)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1591372/logs/1289)||
|[1581470](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1581470)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1581470/logs/1300)||
|[1581235](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1581235)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1581235/logs/1254)||
|[1579251](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1579251)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1579251/logs/1284)||
|[1576881](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1576881)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1576881/logs/1273)||
|[1570294](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1570294)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1570294/logs/1276)||
|[1563592](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1563592)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1563592/logs/1346)||
|[1563522](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1563522)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1563522/logs/1362)||
|[1561283](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1561283)|dotnet/runtime|Send to Helix|[Log](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_apis/build/builds/1561283/logs/1285)||
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|2|10|

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1511237
**Error message validated:** `[The settings file in the tool's NuGet package is invalid: Settings file 'DotnetToolSettings.xml' was not found in the package. Tool 'microsoft.dotnet.xharness.cli' failed to install.`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 7/16/2026 5:14:37 PM UTC

Start with the failing Send to Helix step and the referenced .packages/microsoft.dotnet.helix.sdk/8.0.0-beta.26278.3/tools/xharness-runner/XHarnessRunner.targets and src/libraries/sendtohelixhelp.proj. Review the build logs for the xharness installation and determine why DotnetToolSettings.xml is absent from the tool package. Done means the xharness tool installs successfully and the linux_bionic-arm64 Send to Helix leg passes.

Written by the indexing model from the issue text.