Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Tests/Unit/Client/MockHttpMessageHandler.cs

@@ -0,0 +1,56 @@
+using System.Net;
+using System.Net.Http;
+using System.Text;
+using System.Text.Json;
+
+namespace DocumentService.Tests.Unit.Client;
+
+/// <summary>
+/// Reusable fake <see cref="HttpMessageHandler"/> for unit-testing HTTP clients.
+/// Captures the outgoing request and returns the configured response.
+/// </summary>
+internal sealed class MockHttpMessageHandler : HttpMessageHandler
+{
+    private readonly HttpResponseMessage _response;
+
+    /// <summary>The last request that was sent through this handler.</summary>
+    public HttpRequestMessage? LastRequest { get; private set; }
+
+    public MockHttpMessageHandler(HttpResponseMessage response)
+    {
+        _response = response;
+    }
+
+    // ?? convenience factories ????????????????????????????????????????????????
+
+    /// <summary>Creates a handler that returns 200 OK with a JSON-serialised body.</summary>
+    public static MockHttpMessageHandler ReturningJson<T>(T body, HttpStatusCode status = HttpStatusCode.OK)
+    {
+        var json = JsonSerializer.Serialize(body, new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.CamelCase });
+        var response = new HttpResponseMessage(status)
+        {
+            Content = new StringContent(json, Encoding.UTF8, "application/json")
+        };
+        return new MockHttpMessageHandler(response);
+    }
+
+    /// <summary>Creates a handler that returns 200 OK with raw bytes as the body.</summary>
+    public static MockHttpMessageHandler ReturningBytes(byte[] bytes, string mediaType = "application/octet-stream", HttpStatusCode status = HttpStatusCode.OK)
+    {
+        var response = new HttpResponseMessage(status)
+        {
+            Content = new ByteArrayContent(bytes) { Headers = { ContentType = new(mediaType) } }
+        };
+        return new MockHttpMessageHandler(response);
+    }
+
+    /// <summary>Creates a handler that returns the given status code with no body.</summary>
+    public static MockHttpMessageHandler ReturningStatus(HttpStatusCode status)
+        => new(new HttpResponseMessage(status));
+
+    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        LastRequest = request;
+        return Task.FromResult(_response);
+    }
+}