[ci-scan] Test failure: tracing/eventpipe/nonlossyblockenv drops events in non-lossy block mode

## Build Information
Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1588980
Build error leg or test failing: coreclr Pri1 Runtime Tests Run linux x64 checked - tracing (runtime-interpreter, def 316)

KBE authoring guidance (ci-failure-scan)

- `Error Details` is for readers. Paste the full exception, stack trace, or build error excerpt so the failure is understandable without opening the raw log.
- Build Analysis parses only the single JSON block under `Error Message`.
- `ErrorMessage` is a case-sensitive ordinal `String.Contains` substring copied verbatim from the failing log.
- Set `BuildRetry` to `true` only for a clear infrastructure retry case. `ExcludeConsoleLog` disables Helix console-log scanning.

## Error Details

The `tracing` runtime test's `eventpipe/nonlossyblockenv` scenario fails on the interpreter pipeline: with `DOTNET_EventPipeBufferingMode=1` (block) + streaming FILESTREAM, the trace reports dropped events although block mode should be lossless. Fails across all scanned legs (linux/osx/windows, x64/arm64) and reproduces in source build 1588980 and following builds 1592662 and 1585871, so the signature is stable.

```
-- Positive scenario: Block + streaming FILESTREAM --
[tracee][stdout] [tracee] EventSource enabled = True; emitting 50000 events.
Observed 50000 'NonLossyBlockEnvEventSource' events; EventsLost = 270.
FAILED: expected zero dropped events, but the trace reported 270.
==TEST FINISHED: FAILED!==
at Xunit.Assert.True(Nullable`1 condition, String userMessage)
at TestLibrary.OutOfProcessTest.RunOutOfProcessTest(String assemblyPath, String testPathPrefix)
Failed test: tracing/eventpipe/nonlossyblockenv/nonlossyblockenv/nonlossyblockenv.cmd
```

## Error Message

```json
{
"ErrorMessage": [
"FAILED: expected zero dropped events, but the trace reported",
"Failed test: tracing/eventpipe/nonlossyblockenv/nonlossyblockenv/nonlossyblockenv.cmd"
],
"ErrorPattern": "",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

Agentic workflow metadata (ci-failure-scan)

Workflow artifact: ci-failure-scan
Artifact kind: kbe-verification
Verified match count: 2 hits in failure.log

---
Filed by [`ci-failure-scan`](https://github.com/dotnet/runtime/blob/main/.github/workflows/ci-failure-scan.md) (detection only). [`ci-failure-fix`](https://github.com/dotnet/runtime/blob/main/.github/workflows/ci-failure-fix.md) walks open `[ci-scan]` KBEs and either opens a small fix PR or comments here to loop in owners — it never disables the test.

> [!NOTE]
> This issue was generated by an AI agent (GitHub Copilot / ci-failure-scan workflow).

> Generated by [CI Outer-Loop Failure Scanner](https://github.com/dotnet/runtime/actions/runs/34729903269) · opus48 · 996.9 AIC · ⌖ 16.7 AIC · ⊞ 19.7K · [◷](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

|Build|Repository|Test|Pull Request|
|---|---|---|---|
|[1597012](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1597012)|dotnet/runtime|[tracing/eventpipe/nonlossyblockenv/nonlossyblockenv/nonlossyblockenv.cmd](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1597012&view=ms.vss-test-web.build-test-results-tab&runId=44087580&resultId=115085)||
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|1|1|1|