test: tests Go para funciones email SMTP

Tests para builders puros (build_html, build_text, with_attachment),
template_render, smtp_connect y smtp_send con mock TCP server.
Todos los tests pasan: 18 casos cubriendo pureza, inmutabilidad y envio SMTP.

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

functions/infra/email_template_render_test.go

@@ -0,0 +1,46 @@
+package infra
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestEmailTemplateRender(t *testing.T) {
+	t.Run("renderiza template simple con datos", func(t *testing.T) {
+		got, err := EmailTemplateRender("Hola {{.Name}}", map[string]any{"Name": "Alice"})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if got != "Hola Alice" {
+			t.Errorf("got %q, want %q", got, "Hola Alice")
+		}
+	})
+
+	t.Run("sustituye multiples variables", func(t *testing.T) {
+		tmpl := "Pedido {{.OrderID}} para {{.Customer}} listo."
+		got, err := EmailTemplateRender(tmpl, map[string]any{"OrderID": "ORD-42", "Customer": "Bob"})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if !strings.Contains(got, "ORD-42") || !strings.Contains(got, "Bob") {
+			t.Errorf("got %q, expected OrderID and Customer", got)
+		}
+	})
+
+	t.Run("error en template invalido", func(t *testing.T) {
+		_, err := EmailTemplateRender("{{.Unclosed", nil)
+		if err == nil {
+			t.Error("expected error for invalid template, got nil")
+		}
+	})
+
+	t.Run("template vacio retorna string vacio", func(t *testing.T) {
+		got, err := EmailTemplateRender("", nil)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if got != "" {
+			t.Errorf("got %q, want empty string", got)
+		}
+	})
+}