Added a NUnit test project for the whole solution

app/Tests/TestHost.cs

@@ -0,0 +1,26 @@
+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;
+    }
+}