Skip UseFileServer when wwwroot is missing.

Local Aspire has no published client folder; the middleware warned on every boot.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/HSchool.Server.Tests/ClientStaticFilesTests.cs

@@ -0,0 +1,28 @@
+namespace HSchool.Server.Tests;
+
+public class ClientStaticFilesTests
+{
+    [Fact]
+    public void CanServe_IsFalseWhenTheDirectoryIsMissing()
+    {
+        var missing = Path.Combine(Path.GetTempPath(), "h-school-no-wwwroot-" + Guid.NewGuid().ToString("N"));
+
+        Assert.False(ClientStaticFiles.CanServe(missing));
+        Assert.False(ClientStaticFiles.CanServe(null));
+        Assert.False(ClientStaticFiles.CanServe(""));
+    }
+
+    [Fact]
+    public void CanServe_IsTrueWhenTheDirectoryExists()
+    {
+        var directory = Directory.CreateTempSubdirectory("h-school-wwwroot-");
+        try
+        {
+            Assert.True(ClientStaticFiles.CanServe(directory.FullName));
+        }
+        finally
+        {
+            directory.Delete(recursive: true);
+        }
+    }
+}