WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck fails with unexpected exception

```
System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck [FAIL]
Assert.Throws() Failure: Exception type was not an exact match
Expected: typeof(System.Net.WebSockets.WebSocketException)
Actual: typeof(System.TimeoutException)
---- System.TimeoutException : The operation has timed
```

## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1586944
Build error leg or test failing: System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck
Pull request: https://github.com/dotnet/runtime/pull/130556

## 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": ["ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck","System.TimeoutException : The operation has timed"],
"ErrorPattern": "",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1586944
**Error message validated:** `[ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck System.TimeoutException : The operation has timed`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 9/8/2026 9:03:42 PM UTC

### Report

|Build|Repository|Test|Pull Request|
|---|---|---|---|
|[1596363](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1596363)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1596363&view=ms.vss-test-web.build-test-results-tab&runId=44070036&resultId=123177)|dotnet/runtime#133907|
|[1595488](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1595488)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1595488&view=ms.vss-test-web.build-test-results-tab&runId=44051276&resultId=116107)|dotnet/runtime#133597|
|[1594919](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594919)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594919&view=ms.vss-test-web.build-test-results-tab&runId=44030820&resultId=146253)||
|[1594357](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594357)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1594357&view=ms.vss-test-web.build-test-results-tab&runId=44011966&resultId=135137)|dotnet/runtime#133803|
|[1593809](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1593809)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1593809&view=ms.vss-test-web.build-test-results-tab&runId=43993350&resultId=140322)||
|[1592793](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592793)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592793&view=ms.vss-test-web.build-test-results-tab&runId=43969148&resultId=128875)|dotnet/runtime#133698|
|[1592299](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592299)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592299&view=ms.vss-test-web.build-test-results-tab&runId=43955162&resultId=132130)||
|[1592296](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592296)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592296&view=ms.vss-test-web.build-test-results-tab&runId=43954732&resultId=130881)||
|[1592259](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592259)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592259&view=ms.vss-test-web.build-test-results-tab&runId=43954540&resultId=147502)|dotnet/runtime#133656|
|[1592224](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592224)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1592224&view=ms.vss-test-web.build-test-results-tab&runId=43952438&resultId=111044)|dotnet/runtime#133494|
|[1591897](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591897)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591897&view=ms.vss-test-web.build-test-results-tab&runId=43942916&resultId=134743)|dotnet/runtime#133654|
|[1591180](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591180)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1591180&view=ms.vss-test-web.build-test-results-tab&runId=43923598&resultId=158179)||
|[1588576](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588576)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588576&view=ms.vss-test-web.build-test-results-tab&runId=43846966&resultId=138175)||
|[1588088](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588088)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588088&view=ms.vss-test-web.build-test-results-tab&runId=43828744&resultId=146693)||
|[1586944](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1586944)|dotnet/runtime|[System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1586944&view=ms.vss-test-web.build-test-results-tab&runId=43797194&resultId=147112)|dotnet/runtime#130556|
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|1|14|15|

Start with System.Net.WebSockets.Tests.WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck and the linked build results. Compare the expected WebSocketException with the observed TimeoutException, then review the related pull requests listed in the report. Done means the test no longer intermittently times out and consistently reports the expected behavior.

Written by the indexing model from the issue text.