refactor(tests): DatabaseCollection centraliza ICollectionFixture<SqlTestFixture>

Registra la colección "Database" con SqlTestFixture como fixture compartido
para Application.Tests (elimina el ctor-con-string inline en cada test class).
Agrega Using global a ambos proyectos para evitar usings por archivo.

tests/SIGCM2.Application.Tests/DatabaseCollection.cs

@@ -0,0 +1,15 @@
+using SIGCM2.TestSupport;
+using Xunit;
+
+namespace SIGCM2.Application.Tests;
+
+/// <summary>
+/// Declares the "Database" xUnit collection backed by a single shared SqlTestFixture.
+/// All test classes decorated with [Collection("Database")] share one fixture instance
+/// per test run — eliminating concurrent Respawner collisions.
+/// </summary>
+[CollectionDefinition("Database")]
+public sealed class DatabaseCollection : ICollectionFixture<SqlTestFixture>
+{
+    // Intentionally empty: this class only exists to declare the collection/fixture binding.
+}