feat(rules): add "on: default" rule syntax for default rule

- Add OnDefault rule type that matches when no other rules match
- Add validation to prevent multiple default rules
- Fix typo: extension → extensions in route config JSON tag

internal/route/rules/rules_test.go

@@ -0,0 +1,52 @@
+package rules
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/yusing/godoxy/internal/serialization"
+)
+
+func TestRulesValidate(t *testing.T) {
+	tests := []struct {
+		name  string
+		rules string
+		want  error
+	}{
+		{
+			name: "no default rule",
+			rules: `
+- name: rule1
+  on: header Host example.com
+  do: pass
+      `,
+		},
+		{
+			name: "multiple default rules",
+			rules: `
+- name: default
+  do: pass
+- name: rule1
+  on: default
+  do: pass
+      `,
+			want: ErrMultipleDefaultRules,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var rules Rules
+			convertible, err := serialization.ConvertString(strings.TrimSpace(tt.rules), reflect.ValueOf(&rules))
+			require.True(t, convertible)
+
+			if tt.want == nil {
+				assert.NoError(t, err)
+				return
+			}
+			assert.ErrorIs(t, err, tt.want)
+		})
+	}
+}