Drop the ObjectModel package reference from PlatformServices (Phase 7 capstone)

[Evangelink]

Phase 7 (capstone) — drop the ObjectModel package reference

The final slice of the initiative to make Microsoft.VisualStudio.TestPlatform.MSTestAdapter.PlatformServices platform-agnostic. After this, PlatformServices no longer references the VSTest object model (Microsoft.TestPlatform.ObjectModel) at all — every VSTest dependency now lives in the MSTest.TestAdapter layer above it. Strict byte-for-byte, no behavior change.

Production changes

1. Remove the Microsoft.TestPlatform.ObjectModel PackageReference from MSTestAdapter.PlatformServices.csproj.
2. RunConfigurationSettings was the last consumer of a transitively-provided VSTest package: it used PlatformAbstractions' PlatformApartmentState enum {MTA, STA} to parse ExecutionThreadApartmentState. Replaced with a local internal enum ApartmentStateSetting { MTA, STA } of the same shape (same member names and order — MTA=0, STA=1), preserving the exact Enum.TryParse → STA/MTA → System.Threading.ApartmentState / else-throw behavior byte-for-byte on both the runsettings-XML and config paths. The member order is load-bearing (Enum.TryParse accepts numeric strings "0"/"1", so the numbering must match) and is commented as such. (Parsing directly to System.Threading.ApartmentState would change the handling of the Unknown value — it has that extra member — so a faithful 2-member local enum is required. The property type stays the BCL System.Threading.ApartmentState; only the parse-enum is neutralized.)
3. Add a direct System.Configuration framework reference on .NET Framework. ConfigurationManager/ConfigurationElementCollection (used by TestDataSource) were previously pulled in transitively via the object-model package; System.Configuration is a framework assembly, so it is now referenced directly.

Result: PlatformServices is fully platform-neutral

The compiled MSTestAdapter.PlatformServices assembly has zero references to any Microsoft.*.TestPlatform.* assembly on every real TFM (net462/net8.0/net9.0 + windows variants), verified via assembly metadata.

Guard test (the finish line)

ObjectModelDecouplingTests asserts the compiled PlatformServices assembly references no assembly whose name contains "TestPlatform" — catching ObjectModel, PlatformAbstractions, CoreUtilities, etc. (MSTest's own framework is MSTest.TestFramework, which doesn't match). This locks the platform-agnostic contract permanently.

Test-project fix

PlatformServices.Desktop.IntegrationTests uses ObjectModel's XmlRunSettingsUtilities directly (previously transitive through the PlatformServices project reference), so it gets its own direct Microsoft.TestPlatform.ObjectModel PackageReference. Test projects may reference the object model; only the production assembly must be neutral.

Verification

• All real TFMs build 0-warning (UWP builds via full msbuild in CI).
• Compiled dll: zero TestPlatform references (net462 + net8.0, metadata-verified).
• MSTestAdapter.PlatformServices.UnitTests: 936/936 (net462), 898/898 (net8.0) — includes the new guard test and the STA/MTA parsing tests on both the runsettings-XML and config paths.
• PlatformServices.Desktop.IntegrationTests: 15/15.
• MSTestAdapter.UnitTests: 21/21. MSTest.TestAdapter and MSTest.IntegrationTests build clean.
• Expert-reviewer pass.

Stacking

Stacks on #9632 (Phase 6e-4c3); base branch dev/amauryleve/vstest-decoupling-suspendcoverage. Review/merge after the earlier PRs in the chain reach the base. Do not squash-rebase the base.