Added a NUnit test project for the whole solution

app/Tests/Models/TestHarnessTests.cs

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+using AIStudio.Provider;
+using AIStudio.Settings;
+
+namespace AIStudio.Tests.Models;
+
+/// <summary>
+/// Checks the test harness itself, before any test states something about the app.
+/// </summary>
+/// <remarks>
+/// Three things have to hold before a capability test can mean anything: the app assembly is
+/// referenced, this project counts as a friend assembly, and the assembly-wide setup has run. When
+/// one of them is missing, the failure looks like a broken rule rather than a broken harness, which
+/// is an expensive detour. These two tests make the difference visible right away.
+///
+/// Note the fully written type name below. AIStudio.Provider is a namespace and AIStudio.Settings
+/// .Provider is a type; inside a namespace under AIStudio, the namespace wins the lookup. That is a
+/// property of the app's own naming, not of the tests.
+/// </remarks>
+[TestFixture]
+public sealed class TestHarnessTests
+{
+    [Test]
+    public void TheStaticApplicationStateIsAvailable()
+    {
+        //
+        // Settings.Provider initializes a static logger from Program.LOGGER_FACTORY. Touching it
+        // without the assembly-wide setup throws a TypeInitializationException.
+        //
+        Assert.That(AIStudio.Settings.Provider.NONE.UsedLLMProvider, Is.EqualTo(LLMProviders.NONE));
+    }
+
+    [Test]
+    public void TheCapabilityApiOfTheAppIsReachable()
+    {
+        var capabilities = LLMProviders.OPEN_AI.GetModelCapabilities(new Model("gpt-5.1", null));
+
+        Assert.That(capabilities, Does.Contain(Capability.FUNCTION_CALLING));
+    }
+}

app/Tests/TestHost.cs

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+using Microsoft.Extensions.Logging.Abstractions;
+
+//
+// Deliberately without a namespace: NUnit then applies this fixture to the whole assembly, so every
+// test -- the ones written today and the ones written later in some other folder -- starts with the
+// static state below already in place. A second setup fixture would only ever be needed for state
+// that must not leak between areas.
+//
+namespace AIStudio.Tests;
+
+[SetUpFixture]
+public sealed class TestHost
+{
+    [OneTimeSetUp]
+    public void PrepareStaticApplicationState()
+    {
+        //
+        // A number of types in the app hold a static logger field that is initialized from
+        // Program.LOGGER_FACTORY, among them Settings.Provider. The app assigns that factory while
+        // Kestrel comes up; in a test process nobody does, so it stays null and the first touch of
+        // such a type dies inside its type initializer -- before a single assertion runs. A factory
+        // that writes nowhere is all it takes to get past that.
+        //
+        Program.LOGGER_FACTORY = NullLoggerFactory.Instance;
+    }
+}

app/Tests/Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net9.0</TargetFramework>
+        <LangVersion>latest</LangVersion>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+        <RootNamespace>AIStudio.Tests</RootNamespace>
+        <IsPackable>false</IsPackable>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="coverlet.collector" Version="10.0.1" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.10.0" />
+        <!--
+            Pinned below 4.6.0 on purpose, verified by bisecting 4.4.0, 4.5.0, 4.5.1, 4.6.0 and 4.6.1
+            against the .NET 9 SDK (9.0.120, C# 13): from 4.6.0 on, the compiler no longer picks
+            Assert.That<TActual>(TActual, IResolveConstraint, ...) for a plain value. 4.6.0 added
+            Assert.That(Action, ...) and Assert.That<T>(Func<T>, ...) alongside it, and the value
+            overload drops out of the candidate set, so even Assert.That(1, Is.EqualTo(1)) fails with
+            "argument 1: cannot convert int to System.Action", a message that points nowhere near the
+            real cause. Re-test the newest version whenever the .NET SDK is raised.
+        -->
+        <PackageReference Include="NUnit" Version="4.5.1" />
+        <PackageReference Include="NUnit.Analyzers" Version="4.14.0">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+        <PackageReference Include="NUnit3TestAdapter" Version="6.3.0" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\MindWork AI Studio\MindWork AI Studio.csproj" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <Using Include="NUnit.Framework" />
+    </ItemGroup>
+
+</Project>