Add a cross-process TRX/TCM + data-collector property parity regression test for the MSTest execution pipeline

[Evangelink]

Summary

The initiative to make MSTestAdapter.PlatformServices platform-agnostic (removing its Microsoft.TestPlatform.ObjectModel dependency) has as its #1 invariant that cross-process discover→execute fidelity is preserved byte-for-byte — specifically the TRX / TCM (Test Case Management) properties and data-collector-injected custom properties that the host attaches to test cases and results in cross-process (VSTest host) scenarios.

Today there is no dedicated cross-process regression test that asserts this parity. The fidelity of the phases that touched result recording rests on:

• identical-handle-resolution reasoning (normal results record against the original host TestCase via the memoized UnitTestElement.GetOrCreateHostTestCase() handle; cleanup-failure results use AssociatedUnitTestElement), and
• the in-process MSTest.IntegrationTests suite (which drives discover→run through the engine but does not exercise the cross-process host that injects TCM / data-collector properties).

This is acceptable per-PR, but the gap should be explicit and tracked rather than implicit.

Proposed work

Add an acceptance/integration test that:

1. Runs the MSTest execution pipeline cross-process (via the VSTest host / vstest.console) against a small test asset.
2. Injects host-side TCM properties (ExecutionId/ParentExecId/TestCaseHierarchy/TCM Id/SuiteId/ConfigurationId, etc.) and, where feasible, a data collector that attaches custom result properties/attachments.
3. Asserts the produced TRX (and result properties) are byte-for-byte identical to a captured baseline — i.e. that the neutralized engine does not drop or alter host-injected properties on normal results.

Notes

• This is acceptance-harness work and should not block the decoupling phases; capture it here so a follow-up (or Phase 7) can close it.
• Phases that would have benefited and whose fidelity currently rests on the reasoning above: Abstract VSTest execution input in PlatformServices (Phase 4 of platform-agnostic effort) #9572 (execution input) and Neutralize test result recording in PlatformServices (Phase 6b) #9579 (result recording).

Category

Task (test gap / regression-coverage follow-up).