Neutralize run-settings input in PlatformServices host layer (Phase 6c2)

[Evangelink]

Phase 6c2 of the PlatformServices platform-agnostic initiative

Part of removing the VSTest object model (Microsoft.TestPlatform.ObjectModel) dependency from MSTestAdapter.PlatformServices, moving VSTest coupling up into MSTest.TestAdapter.

This phase removes the VSTest IRunSettings type: it threads a neutral settings-XML string through the isolation-host layer instead. Only IRunSettings.SettingsXml (a string) was ever read across PlatformServices, so this is a mechanical, byte-for-byte substitution.

What changed

• IPlatformServiceProvider.CreateTestSourceHost(string, IRunSettings?) → (string, string? settingsXml).
• TestSourceHost (both ctors, incl. the netfx AppDomain path) and AssemblyEnumeratorWrapper.GetTests/GetTestsInIsolation → string? settingsXml.
• TestExecutionManager.CacheSessionParameters takes the settings-XML string directly.
• Callers extract runContext?.RunSettings?.SettingsXml / discoveryContext?.RunSettings?.SettingsXml at the point they already held the (still-VSTest) run/discovery context.

No behavior change

The AppDomain DisableAppDomain decision and run-parameter caching read the same settings XML as before. The one guard (runSettings != null && … → settingsXml is not null && …) is byte-for-byte for all realistic inputs and matches the existing DeploymentUtilityBase precedent (expert-reviewer confirmed the only theoretical divergence is unreachable). IRunSettings is now fully gone from PlatformServices code (only a doc-comment mention remains). The remaining IRunContext/IDiscoveryContext usage is the test-case filter, deferred to the filter sub-phase.

Verification

• Full build green across all TFMs (net462, net8.0, net9.0, UWP, WinUI).
• MSTestAdapter.PlatformServices.UnitTests: 897 (net8.0) / 935 (net462); MSTest.IntegrationTests: 47 pass / 1 skip; PlatformServices.Desktop.IntegrationTests: 15 pass (exercises the AppDomain assembly-resolution-from-runsettings path — validates the settings-xml threads correctly through the child domain).
• Expert MSTest/MTP reviewer: no material findings; gave the AppDomain guard change a definitive byte-for-byte verdict.

Base

• Base = dev/amauryleve/vstest-decoupling-base, which already contains Phase 3 (Abstract VSTest discovery sink in PlatformServices (Phase 3 of platform-agnostic effort) #9566), 4 (Abstract VSTest execution input in PlatformServices (Phase 4 of platform-agnostic effort) #9572), 6a (Neutralize deployment input in PlatformServices (Phase 6a of platform-agnostic effort) #9576), 6b (Neutralize test result recording in PlatformServices (Phase 6b) #9579), and 6c (Neutralize test message logging in PlatformServices execution engine (Phase 6c) #9585). Developed stacked on 6c; 6c has since merged into the base, so the diff here is 6c2 only (11 files).

Remaining work

• Filter internals (IRunContext/IDiscoveryContext/ITestCaseFilterExpression → boundary; folds in Avoid double TestCase construction when filtering during discovery/execution (platform-agnostic refactor follow-up) #9568).
• Relocate remaining bridges + the deep UnitTestElement↔TestCase conversion + EngineConstants VSTest properties.
• Remove the Microsoft.TestPlatform.ObjectModel PackageReference + guard test.