Added abstraction layeer IHtmlLoader. Finished reorganizing test project folder structure.

MangaReader.Core/Http/HttpService.cs

@@ -0,0 +1,25 @@
+namespace MangaReader.Core.Http;
+
+public class HttpService(HttpClient httpClient) : IHttpService
+{
+    public Task<string> GetStringAsync(string url, CancellationToken cancellationToken)
+        => GetStringAsync(url, new Dictionary<string, string>(),  cancellationToken);
+
+    public async Task<string> GetStringAsync(string url, IDictionary<string, string> headers, CancellationToken cancellationToken)
+    {
+        using HttpRequestMessage request = new(HttpMethod.Get, url);
+
+        foreach (KeyValuePair<string, string> header in headers)
+        {
+            request.Headers.TryAddWithoutValidation(header.Key, header.Value);
+        }
+
+        //httpClient.DefaultRequestHeaders.UserAgent.ParseAdd("MangaReader/1.0");
+        httpClient.DefaultRequestHeaders.UserAgent.ParseAdd("Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:139.0) Gecko/20100101 Firefox/139.0");
+
+        using HttpResponseMessage response = await httpClient.SendAsync(request, cancellationToken);
+        response.EnsureSuccessStatusCode();
+
+        return await response.Content.ReadAsStringAsync(cancellationToken);
+    }
+}