feat(infra): funciones impuras Go para carga de env/config (dotenv, env_require, config_from_env, config_from_file)

- dotenv_load: parser .env con no-sobreescritura y soporte de comillas
- env_require: os.Getenv con error descriptivo fail-fast
- env_require_all: verifica multiples vars y lista todas las faltantes
- config_from_env: reflection sobre struct tags env/default/required/secret, 5 tipos soportados
- config_from_file: JSON via stdlib, YAML stub con not-implemented
- 25 tests unitarios, todos PASS

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

functions/infra/config_from_file_test.go

@@ -0,0 +1,66 @@
+package infra
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestConfigFromFile(t *testing.T) {
+	t.Run("carga JSON valido en struct", func(t *testing.T) {
+		type Cfg struct {
+			Host string `json:"host"`
+			Port int    `json:"port"`
+		}
+		content := `{"host":"testhost","port":5432}`
+		dir := t.TempDir()
+		p := filepath.Join(dir, "config.json")
+		if err := os.WriteFile(p, []byte(content), 0o600); err != nil {
+			t.Fatal(err)
+		}
+
+		var c Cfg
+		if err := ConfigFromFile(p, &c); err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if c.Host != "testhost" || c.Port != 5432 {
+			t.Errorf("unexpected: %+v", c)
+		}
+	})
+
+	t.Run("JSON invalido retorna error", func(t *testing.T) {
+		dir := t.TempDir()
+		p := filepath.Join(dir, "bad.json")
+		os.WriteFile(p, []byte("{not valid json}"), 0o600)
+
+		var target map[string]any
+		err := ConfigFromFile(p, &target)
+		if err == nil {
+			t.Error("expected error for invalid JSON")
+		}
+	})
+
+	t.Run("archivo inexistente retorna error", func(t *testing.T) {
+		var target map[string]any
+		err := ConfigFromFile("/nonexistent/config.json", &target)
+		if err == nil {
+			t.Error("expected error for missing file")
+		}
+	})
+
+	t.Run("extension YAML retorna not implemented", func(t *testing.T) {
+		var target map[string]any
+		err := ConfigFromFile("config.yaml", &target)
+		if err == nil {
+			t.Error("expected not-implemented error for YAML")
+		}
+	})
+
+	t.Run("extension desconocida retorna error", func(t *testing.T) {
+		var target map[string]any
+		err := ConfigFromFile("config.toml", &target)
+		if err == nil {
+			t.Error("expected error for unsupported extension")
+		}
+	})
+}