[WIP] Stabilize ReactiveUI.Tests by isolating static state and de-parallelizing affected tests

[Copilot]

• Audit ReactiveUI.Tests for static/global state usage (RxApp, Locator, MessageBus, ViewLocator, ModeDetector)
• Create helper scope classes to snapshot and restore static state
  • RxAppSchedulersScope for RxApp schedulers
  • LocatorScope for Splat dependency resolver
  • MessageBusScope for MessageBus.Current
• Mark affected test fixtures as [NonParallelizable]
  • MessageBusTest.cs (uses Locator.CurrentMutable, MessageBus.Current)
  • RandomTests.cs (uses RxApp, ViewLocator.Current, Locator, MessageBus.Current)
  • RxAppTest.cs (uses RxApp.MainThreadScheduler)
  • ReactiveCommandTest.cs (uses RxApp.EnsureInitialized)
  • PocoObservableForPropertyTests.cs (uses RxApp.EnsureInitialized)
  • Platform-specific tests (RoutedViewHostTests, ViewModelViewHostTests, etc.)
  • AwaiterTest.cs (uses RxApp.TaskpoolScheduler)
• Apply helper scopes in test setup/teardown where needed
• Document rationale for non-parallelizable fixtures
• Verify tests run deterministically (on Windows environment)

Note: Build environment requires Windows with .NET 8/9/10 SDKs. Current Linux environment cannot build/test but can prepare code changes.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Stabilize ReactiveUI.Tests by isolating static state and de-parallelizing affected tests</issue_title>
<issue_description>Goal: Make tests deterministic. Audit tests for usage of static/global state and ensure each such test:

1. does not run in parallel, and
2. restores all mutated static state to its prior values (via helper scopes/fixtures as needed).

This work is blocked unless the build & test environment is set up exactly as specified below. Follow the pre-flight steps first.

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🔒 Pre-flight: Build Environment Requirements (must be completed before any code changes)

Cloning the Repository

CRITICAL: Perform a full, recursive clone. A shallow clone (--depth 1) will fail because the build reads git version info.

# Full recursive clone
git clone --recursive https://github.com/reactiveui/reactiveui.git
cd reactiveui

Required Tools

• .NET SDKs: 8.0, 9.0, and 10.0

CRITICAL: Install all three SDKs via the official script:

# Download the installation script
Invoke-WebRequest -Uri https://dot.net/v1/dotnet-install.ps1 -OutFile dotnet-install.ps1

# Install .NET 8 SDK
./dotnet-install.ps1 -Channel 8.0 -InstallDir ./.dotnet

# Install .NET 9 SDK
./dotnet-install.ps1 -Channel 9.0 -InstallDir ./.dotnet

# Install .NET 10 SDK
./dotnet-install.ps1 -Channel 10.0 -InstallDir ./.dotnet

(Tip: ensure ./.dotnet is on PATH for this shell session, or call it explicitly via ./.dotnet/dotnet.)

Solution Files

• Main solution: src/ReactiveUI.sln
• Integration test solutions live under integrationtests/ (not required for this task)

🛠️ Build & Test Commands (Windows PowerShell or CMD, repository root)

CRITICAL: dotnet workload restore must be run from /src before building.

dotnet --info

# CRITICAL: restore workloads from /src
cd src
dotnet workload restore
cd ..

# Restore packages
dotnet restore src/ReactiveUI.sln

# Build (requires Windows for platform targets)
dotnet build src/ReactiveUI.sln -c Release -warnaserror

# Run tests (includes AOT tests requiring .NET 9.0)
dotnet test src/ReactiveUI.sln -c Release --no-build

Note: Building on Linux/macOS will fail due to net*-windows targets and AOT tests. Use Windows.

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🧭 Context & Example Failure

We’re seeing intermittent failures tied to static/global state. Example:

Failed AutoPersistDoesntWorkOnNonDataContractClasses
ReactiveUI.UnhandledErrorException ... Could not find a ICreatesObservableForProperty ...
... your service locator is probably broken ...
at ReactiveUI.RxApp.<>c__DisplayClass9_0.<.cctor>b__2() ...

This indicates tests mutate or rely on global/static state (e.g., RxApp, service locator, schedulers, message bus, etc.) without restoring it. Running in parallel amplifies the flakiness.

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✅ What to do

1) Discover tests that touch static/global state

Search for usages of well-known static entry points or singletons anywhere under src/ReactiveUI.Tests:

• ReactiveUI: RxApp., MessageBus., SuspensionHost., Interaction., ModeDetector., LogHost.
• Splat / DI: Locator., Locator.Current, Locator.CurrentMutable, DependencyResolver, Splat.Locator
• Any other static members that mutate shared state (caches, registries, global flags, default schedulers, etc.)

Suggested queries (run at repo root):

git grep -n "RxApp\." -- src/ReactiveUI.Tests
git grep -n "Locator\." -- src/ReactiveUI.Tests
git grep -n "MessageBus\." -- src/ReactiveUI.Tests
git grep -n "SuspensionHost\." -- src/ReactiveUI.Tests
git grep -n "ModeDetector\." -- src/ReactiveUI.Tests
git grep -n "LogHost\." -- src/ReactiveUI.Tests
git grep -n "static " -- src/ReactiveUI.Tests

Output: Build a checklist of test files and specific tests that read or write static/global state.

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2) Make affected tests non-parallelizable (targeted)

For each test fixture or test that touches static/global state, explicitly disable parallelization with NUnit:

• At the fixture level (preferred when multiple tests in a class touch static state):

  using NUnit.Framework;
  
  [TestFixture]
  [NonParallelizable] // or [Parallelizable(ParallelScope.None)]
  public class FooTests { ... }

• Or at the individual test level if only one or two tests are affected:

  [Test, NonParallelizable]
  public void MyTest() { ... }

Do not disable parallelization assembly-wide. Keep the scope as tight as possible.

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3) Introduce helper scopes to snapshot & restore static state

Create small, focused disposable scopes for each area of static state. These should snapshot on construction, and restore on Dispose(). Use them in SetUp/TearDown or in using blocks inside tests.

⚠️ Use the actual APIs present in this codebase—names below are examples. Inspect the concrete properties/me...

Fixes #4165

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