Added a NUnit test project for the whole solution

app/Tests/Models/TestHarnessTests.cs

@@ -0,0 +1,39 @@
+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));
+    }
+}