feat: wip on login endpoint

- added basic REPR pattern
- added basic vert architecture
- implemented basica validation for single test case

tests/FiscalOS.API.Tests/Integration/LoginTests.cs

@@ -0,0 +1,28 @@
+using System.Net.Http.Json;
+
+using Microsoft.AspNetCore.Mvc;
+
+namespace FiscalOS.API.Tests.Integration;
+
+public class LoginTests(TestApi testApi) : IClassFixture<TestApi>
+{
+  private readonly TestApi _testApi = testApi;
+
+  [Fact]
+  public async Task Login_WhenUserSubmitsWithoutUsernameOrPassword_ItShouldReturn400WithProblemDetails()
+  {
+    var client = _testApi.CreateClient();
+
+    var res = await client.PostAsJsonAsync("/login", new { }, TestContext.Current.CancellationToken);
+
+    res.StatusCode.Should().Be(System.Net.HttpStatusCode.BadRequest);
+
+    var problem = await res.Content.ReadFromJsonAsync<ValidationProblemDetails>(TestContext.Current.CancellationToken);
+
+    problem!.Errors.Should().BeEquivalentTo(new Dictionary<string, string[]>()
+    {
+      ["Username"] = ["The Username field is required."],
+      ["Password"] = ["The Password field is required."]
+    });
+  }
+}