feat: tipos TestServer y TestDB para utilidades de testing en Go core

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/core/test_server.go

@@ -0,0 +1,12 @@
+package core
+
+import (
+	"net/http"
+)
+
+// TestServer encapsula un servidor HTTP de test con su cliente y cleanup.
+type TestServer struct {
+	URL     string
+	Client  *http.Client
+	Cleanup func()
+}