Ensure Splat initialization in builder and tests (#4098)

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**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

update, feature

**What is the new behavior?**
<!-- If this is a feature change -->

Added calls to InitializeSplat in ReactiveUIBuilder, builder extension,
and RxApp static constructor to ensure Splat services are registered
before core ReactiveUI services. Updated test classes to call
RxApp.EnsureInitialized in constructors for consistent initialization.
Also updated Splat package version in Directory.Packages.props.

**What might this PR break?**

Part of a major update for V21

**Please check if the PR fulfills these requirements**
- [x] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:

Author: ChrisPulman

src/Directory.Packages.props

@@ -4,7 +4,7 @@
     <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
   </PropertyGroup>
   <PropertyGroup>
-    <SplatVersion>15.5.3</SplatVersion>
+    <SplatVersion>16.0.1</SplatVersion>
     <XamarinAndroidXCoreVersion>1.13.1.4</XamarinAndroidXCoreVersion>
     <XamarinAndroidXLifecycleLiveDataVersion>2.8.4.1</XamarinAndroidXLifecycleLiveDataVersion>
   </PropertyGroup>

src/ReactiveUI.AOTTests/ViewLocatorAOTMappingTests.cs

@@ -12,6 +12,14 @@ namespace ReactiveUI.AOTTests;
 /// </summary>
 public class ViewLocatorAOTMappingTests
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ViewLocatorAOTMappingTests"/> class.
+    /// </summary>
+    public ViewLocatorAOTMappingTests()
+    {
+        RxApp.EnsureInitialized();
+    }
+
     /// <summary>
     /// Map/Resolve with contract and default fallback works.
     /// </summary>

src/ReactiveUI.Builder.Maui.Tests/ReactiveUIBuilderMauiTests.cs

@@ -31,7 +31,6 @@ public void WithCoreServices_AndMaui_Should_Register_All_Services()
         using var locator = new ModernDependencyResolver();
 
         locator.CreateBuilder()
-               .WithCoreServices()
                .WithMaui()
                .Build();
 

src/ReactiveUI.Tests/Commands/ReactiveCommandTest.cs

@@ -18,6 +18,11 @@ namespace ReactiveUI.Tests;
 /// </summary>
 public class ReactiveCommandTest
 {
+    public ReactiveCommandTest()
+    {
+        RxApp.EnsureInitialized();
+    }
+
     /// <summary>
     /// A test that determines whether this instance [can execute changed is available via ICommand].
     /// </summary>

src/ReactiveUI.Tests/Platforms/winforms/DefaultPropertyBindingTests.cs

@@ -17,6 +17,14 @@ namespace ReactiveUI.Tests.Winforms;
 /// </summary>
 public class DefaultPropertyBindingTests
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="DefaultPropertyBindingTests"/> class.
+    /// </summary>
+    public DefaultPropertyBindingTests()
+    {
+        RxApp.EnsureInitialized();
+    }
+
     /// <summary>
     /// Tests Winforms creates observable for property works for textboxes.
     /// </summary>

src/ReactiveUI.Tests/Platforms/wpf/ReactiveUIBuilderWpfTests.cs

@@ -13,6 +13,14 @@ namespace ReactiveUI.Tests.Platforms.Wpf;
 /// </summary>
 public class ReactiveUIBuilderWpfTests
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ReactiveUIBuilderWpfTests"/> class.
+    /// </summary>
+    public ReactiveUIBuilderWpfTests()
+    {
+        RxApp.EnsureInitialized();
+    }
+
     /// <summary>
     /// Test that WPF services can be registered using the builder.
     /// </summary>

src/ReactiveUI/Builder/ReactiveUIBuilder.cs

@@ -38,6 +38,8 @@ public override AppBuilder WithCoreServices()
             return this;
         }
 
+        _resolver.InitializeSplat();
+
         // Immediately register the core ReactiveUI services into the provided resolver.
         var registrations = new Registrations();
 #pragma warning disable IL2067 // Target parameter argument does not satisfy 'DynamicallyAccessedMembersAttribute' in call to target method. The parameter of method does not have matching annotations.

src/ReactiveUI/Mixins/ReactiveUIBuilderExtensions.cs

@@ -26,6 +26,7 @@ public static class ReactiveUIBuilderExtensions
     public static AppBuilder CreateBuilder(this IMutableDependencyResolver resolver)
     {
         resolver.ArgumentNullExceptionThrowIfNull(nameof(resolver));
+        resolver.InitializeSplat();
         var builder = new Builder.ReactiveUIBuilder(resolver);
 
         // Queue core registrations by default so Build() always provides the basics

src/ReactiveUI/RxApp.cs

@@ -84,6 +84,7 @@ static RxApp()
 #if !PORTABLE
         _taskpoolScheduler = TaskPoolScheduler.Default;
 #endif
+        Locator.CurrentMutable.InitializeSplat();
 
         if (!AppBuilder.UsingBuilder)
         {
@@ -94,6 +95,7 @@ static RxApp()
                     return;
                 }
 
+                Locator.CurrentMutable.InitializeSplat();
                 Locator.CurrentMutable.InitializeReactiveUI(PlatformRegistrationManager.NamespacesToRegister);
             });
         }