Add a regression test to validate sample profiling events

[noahfalk]

Follow up on #124019

[Copilot]

Pull request overview

This PR adds a regression test to validate that the SampleProfiler correctly reports the SampleType field in ThreadSample events. The test ensures that when threads are executing managed code, the SampleType is reported as "Managed" (value 2) rather than incorrectly reporting "External" (value 1), which was a regression introduced in .NET 9.

Changes:

• Added a new test project SampleProfilerSampleType that validates ThreadSample events report correct SampleType values
• Test spins doing managed work and verifies at least some samples are marked as Managed type
• Follows the established pattern for EventPipe validation tests in the repository

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/tests/tracing/eventpipe/eventsvalidation/SampleProfilerSampleType.csproj	Test project file with standard configuration for tracing tests (GCStressIncompatible, JitOptimizationSensitive)
src/tests/tracing/eventpipe/eventsvalidation/SampleProfilerSampleType.cs	Regression test that validates ThreadSample events report SampleType=Managed when executing managed code

[noahfalk]

/azp run runtime

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[noahfalk]

/ba-g - This PR already passed CI earlier but everything got reset trying to rebase it at the end. I don't think its meaningful to re-run.