refactor(tests): SqlTestFixture usa TestConnectionStrings; ctor interno para Api.Tests

Agrega ctor parameterless que apunta a SIGCM2_Test_App (requerido por
xUnit ICollectionFixture<T>). El ctor con string se marca internal y
expone via InternalsVisibleTo a SIGCM2.Api.Tests. TestWebAppFactory
apunta a SIGCM2_Test_Api. Se agrega propiedad Connection pública para
que los tests que necesitan queries ad-hoc la usen.

tests/SIGCM2.TestSupport/SqlTestFixture.cs

@@ -16,7 +16,14 @@ public sealed class SqlTestFixture : IAsyncLifetime
     private SqlConnection _connection = null!;
     private Respawner _respawner = null!;
 
-    public SqlTestFixture(string connectionString)
+    /// <summary>Parameterless ctor for xUnit ICollectionFixture — uses SIGCM2_Test_App.</summary>
+    public SqlTestFixture() : this(TestConnectionStrings.AppTestDb) { }
+
+    /// <summary>
+    /// Explicit connection string ctor — used by TestWebAppFactory (same assembly).
+    /// Internal to satisfy xUnit's "single public constructor" rule for ICollectionFixture.
+    /// </summary>
+    internal SqlTestFixture(string connectionString)
     {
         _connectionString = connectionString;
     }
@@ -80,6 +87,13 @@ public sealed class SqlTestFixture : IAsyncLifetime
         await ResetAndSeedAsync();
     }
 
+    /// <summary>
+    /// Exposes the open SqlConnection for tests that need to run ad-hoc queries
+    /// (e.g. seed extra rows, assert history tables). Connection is opened during
+    /// InitializeAsync and closed in DisposeAsync.
+    /// </summary>
+    public SqlConnection Connection => _connection;
+
     public async Task ResetAndSeedAsync()
     {
         await _respawner.ResetAsync(_connection);

tests/SIGCM2.TestSupport/TestWebAppFactory.cs

@@ -13,12 +13,11 @@ namespace SIGCM2.TestSupport;
 
 /// <summary>
 /// WebApplicationFactory for integration tests against SIGCM2.Api.
-/// Uses SIGCM2_Test database (separate from production SIGCM2).
+/// Uses SIGCM2_Test_Api database (isolated from Application.Tests which uses SIGCM2_Test_App).
 /// </summary>
 public sealed class TestWebAppFactory : WebApplicationFactory<Program>, IAsyncLifetime
 {
-    private const string TestConnectionString =
+    private const string TestConnectionString = TestConnectionStrings.ApiTestDb;
-        "Server=TECNICA3;Database=SIGCM2_Test;User Id=desarrollo;Password=desarrollo2026;TrustServerCertificate=True;";
 
     // Resolved once — absolute paths independent of working directory
     private static readonly string RepoRoot = ResolveRepoRoot();