[IsolationSession] Select the C# end-to-end test classes in any namespace

[adpa-ms]

📖 Description

The scheduled isolation-session jobs selected the C# end-to-end tests by fully qualified class name. #1355 moved both classes into the Microsoft.Mxc.Sdk.Tests.V1 namespace, so the filters matched nothing, the C# suite reported that it executed no tests, and all four isolation-session jobs failed.

The suite now selects MxcSandboxIsolationSessionE2ETests and MxcLifecycleE2ETests in any namespace (-class *.<name>), and fails when either class executes no tests.

🔗 References

Follows #1355. Failing nightly: https://github.com/microsoft/mxc/actions/runs/36982494352

🔍 Validation

• With the failing nightly's own test binary on an x64 test machine with the IsolationSession service and MXC_ISO_TESTS_REQUIRED=1: the old filters ran no tests; the new filters ran 9, all passed.
• The new check, given those runs' result XML: it passes the 9-test run, fails the empty run naming both classes, and fails a copy with one class's results removed, which the previous check passed.
• The Scheduled Validation Tests workflow on this branch, limited to the isolation-session entries: all four jobs passed (windows-prerelease-isolation-session and windows-prerelease-26h1, x64 and arm64), each running 9 C# tests: https://github.com/microsoft/mxc/actions/runs/37020708486

✅ Checklist

• Signed the Contributor License Agreement

📋 Issue Type

• Bug fix

Microsoft Reviewers: Open in CodeFlow

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What changed in this PR

Fixes isolation-session scheduled jobs by making C# end-to-end test selection resilient to namespace changes and by failing the job if either required E2E test class runs zero tests.

Changes:

• Update C# xUnit runner filters to select the target test classes in any namespace (-class *.<name>).
• Add XML-based validation to ensure each required class executed at least one passing/failing test, and report failures clearly.

File	Description
scripts/​ci/​run_backend_validation_tests.ps1	Updates isolation-session C# test filtering and adds per-class “no tests executed” validation via xUnit result XML.

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