feat(tests): añade TestConnectionStrings y script de creación de DBs de test

Introduce SIGCM2_Test_App y SIGCM2_Test_Api como bases aisladas para
Application.Tests y Api.Tests respectivamente. TestConnectionStrings.cs
centraliza las connection strings; create-test-api-db.sql documenta
el setup idempotente de ambas bases con COLLATE Modern_Spanish_CI_AS.

tests/SIGCM2.TestSupport/TestConnectionStrings.cs

@@ -0,0 +1,20 @@
+using System.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("SIGCM2.Api.Tests")]
+
+namespace SIGCM2.TestSupport;
+
+/// <summary>
+/// Centralized connection string constants for integration test databases.
+/// Single source of truth — change server/credentials here only.
+/// </summary>
+public static class TestConnectionStrings
+{
+    /// <summary>Used by SIGCM2.Application.Tests via SqlTestFixture (parameterless ctor).</summary>
+    public const string AppTestDb =
+        "Server=TECNICA3;Database=SIGCM2_Test_App;User Id=desarrollo;Password=desarrollo2026;TrustServerCertificate=True;";
+
+    /// <summary>Used by SIGCM2.Api.Tests via TestWebAppFactory.</summary>
+    public const string ApiTestDb =
+        "Server=TECNICA3;Database=SIGCM2_Test_Api;User Id=desarrollo;Password=desarrollo2026;TrustServerCertificate=True;";
+}