Relocate test project: test/yavscTests -> src/Yavsc.Org.Tests

Move the integration test project from the top-level test/ directory into
src/ alongside the projects it tests. Rename the project (and folder) to
Yavsc.Org.Tests to match .NET conventions and reflect that it tests the
Org runtime primarily.

Path changes:
- test/yavscTests/yavscTests.csproj -> src/Yavsc.Org.Tests/Yavsc.Org.Tests.csproj
- All .cs / .json / .resx files moved to their new location
- PostItViewModelTests moved out to the dedicated src/PostIt.Tests project
  (it was unrelated to Org testing)

Build adjustments:
- <ProjectReference> paths shortened (..\..\src\X -> ..\X)
- PostIt project reference removed (covered by its own test project)
- <OutputType>exe added (required by xunit.v3)
- xunit.v3.common and xunit.v3.extensibility.core added to package versions

Solution + sln:
- yavsc.sln Project Name updated to 'Yavsc.Org.Tests' and path updated
- GUID preserved so existing build configs stay valid

Static web assets:
- The CopyStaticWebAssetsManifest target was hard-coding the destination
  filename to 'testhost.staticwebassets.endpoints.json', which worked
  when the assembly was named 'yavscTests'. Now that the assembly name
  is 'Yavsc.Org.Tests', ASP.NET Core's MapStaticAssets() looks for
  'Yavsc.Org.Tests.staticwebassets.endpoints.json' (entry-assembly-based
  resolution). Use $(MSBuildProjectName) so the copy target stays
  correct under any future rename.

src/PostIt.Tests/PostItViewModelTests.cs

@@ -0,0 +1,79 @@
+using System.Collections.Generic;
+using System.Linq;
+using System.Net;
+using System.Net.Http;
+using System.Net.Http.Json;
+using System.Text;
+using System.Text.Json;
+using System.Threading.Tasks;
+using PostIt.Models;
+using PostIt.Services;
+using PostIt.ViewModels;
+using Xunit;
+
+namespace PostIt;
+
+public class PostItViewModelTests
+{
+    [Fact]
+    public void SearchCommand_filters_posts_by_title_article_or_author()
+    {
+        var viewModel = new MainPageViewModel();
+
+        viewModel.Posts.Add(new BlogPost { Id = 1, Title = "First post", Article = "Hello world", AuthorId = "alice" });
+        viewModel.Posts.Add(new BlogPost { Id = 2, Title = "Second post", Article = "Nothing here", AuthorId = "bob" });
+        viewModel.Posts.Add(new BlogPost { Id = 3, Title = "Third post", Article = "Search me", AuthorId = "carol" });
+
+        viewModel.SearchText = "search";
+        viewModel.SearchCommand.Execute(null);
+
+        Assert.Single(viewModel.FilteredPosts);
+        Assert.Equal(3, viewModel.FilteredPosts[0].Id);
+
+        viewModel.SearchText = "bob";
+        viewModel.SearchCommand.Execute(null);
+
+        Assert.Single(viewModel.FilteredPosts);
+        Assert.Equal(2, viewModel.FilteredPosts[0].Id);
+    }
+
+    [Fact]
+    public async Task BlogApiClient_GetPostsAsync_returns_posts_from_api()
+    {
+        var expected = new List<BlogPost>
+        {
+            new() { Id = 1, Title = "Hello" },
+            new() { Id = 2, Title = "World" }
+        };
+
+        var handler = new FakeHttpMessageHandler(HttpStatusCode.OK, JsonSerializer.Serialize(expected));
+        using var client = new HttpClient(handler)
+        {
+            BaseAddress = new System.Uri("http://localhost/")
+        };
+
+        using var apiClient = new BlogApiClient(client);
+        var posts = await apiClient.GetPostsAsync();
+
+        Assert.Equal(2, posts.Count);
+        Assert.Equal("Hello", posts[0].Title);
+    }
+
+    private sealed class FakeHttpMessageHandler : HttpMessageHandler
+    {
+        private readonly HttpResponseMessage _response;
+
+        public FakeHttpMessageHandler(HttpStatusCode statusCode, string content)
+        {
+            _response = new HttpResponseMessage(statusCode)
+            {
+                Content = new StringContent(content, Encoding.UTF8, "application/json")
+            };
+        }
+
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
+        {
+            return Task.FromResult(_response);
+        }
+    }
+}