fix: add motivation config precedence regression test

Codify the article's nested Config scenario to verify YAML base values are overridden by dotenv and finally by process environment variables.

Co-authored-by: Cursor <cursoragent@cursor.com>

motivation_config_test.go

@@ -0,0 +1,58 @@
+package config_test
+
+import (
+	"testing"
+
+	"github.com/eslider/go-config/env"
+	"github.com/eslider/go-config/yaml"
+)
+
+type motivationConfig struct {
+	HTTP struct {
+		Listen string `mapstructure:"listen"`
+		TLS    bool   `mapstructure:"tls"`
+	} `mapstructure:"http"`
+	Database struct {
+		URL string `mapstructure:"url"`
+	} `mapstructure:"database"`
+}
+
+func TestMotivationConfig_YAMLThenEnvThenProcessEnv(t *testing.T) {
+	t.Setenv("HTTP_LISTEN", ":10000")
+	t.Setenv("HTTP_TLS", "true")
+	t.Setenv("DATABASE_URL", "postgres://process-env")
+
+	yamlCfg := yaml.New(yaml.WithBytes([]byte(`
+http:
+  listen: ":8080"
+  tls: false
+database:
+  url: "postgres://yaml"
+`)))
+
+	envCfg := env.New(
+		env.WithBytes([]byte(`
+HTTP_LISTEN=:9090
+DATABASE_URL=postgres://dotenv
+`)),
+		env.WithCurrentEnvironment(),
+	)
+
+	var cfg motivationConfig
+	if err := yamlCfg.Unmarshal(&cfg); err != nil {
+		t.Fatalf("yaml unmarshal: %v", err)
+	}
+	if err := envCfg.Unmarshal(&cfg); err != nil {
+		t.Fatalf("env unmarshal: %v", err)
+	}
+
+	if cfg.HTTP.Listen != ":10000" {
+		t.Fatalf("HTTP.Listen=%q", cfg.HTTP.Listen)
+	}
+	if !cfg.HTTP.TLS {
+		t.Fatalf("HTTP.TLS=%v", cfg.HTTP.TLS)
+	}
+	if cfg.Database.URL != "postgres://process-env" {
+		t.Fatalf("Database.URL=%q", cfg.Database.URL)
+	}
+}