refactor(tests): Application.Tests elimina Respawner inline; usa SqlTestFixture compartido

6 clases que instanciaban Respawner directamente migran a recibir SqlTestFixture
vía ICollectionFixture. 8 clases restantes solo actualizan ConnectionString a
TestConnectionStrings.AppTestDb. Cada clase ahora es responsable únicamente de
sus seeds específicos; la limpieza de la base queda centralizada en el fixture.

tests/SIGCM2.Application.Tests/Infrastructure/Audit/SecurityEventRepositoryTests.cs

@@ -11,8 +11,7 @@ namespace SIGCM2.Application.Tests.Infrastructure.Audit;
 [Collection("Database")]
 public sealed class SecurityEventRepositoryTests : IAsyncLifetime
 {
-    private const string ConnectionString =
-        "Server=TECNICA3;Database=SIGCM2_Test;User Id=desarrollo;Password=desarrollo2026;TrustServerCertificate=True;";
+    private const string ConnectionString = TestConnectionStrings.AppTestDb;
 
     private SqlConnection _connection = null!;
     private SecurityEventRepository _repo = null!;