test: improve coverage for pagination and domain packages

- Add comprehensive cursor pagination tests (95.7% coverage)
- Add domain helper functions tests (StrPtr, DerefStr)
- Add Gender and UserStatus constants tests
- Add User model tests (TableName, default values)
- Overall coverage improved from 53.2% to 53.5%

internal/domain/user_helper_test.go

@@ -0,0 +1,129 @@
+package domain
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestStrPtr 测试 StrPtr 函数
+func TestStrPtr(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected *string
+	}{
+		{
+			name:     "empty string",
+			input:    "",
+			expected: nil,
+		},
+		{
+			name:     "non-empty string",
+			input:    "test@example.com",
+			expected: strPtr("test@example.com"),
+		},
+		{
+			name:     "whitespace string",
+			input:    "   ",
+			expected: strPtr("   "),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := StrPtr(tt.input)
+			if tt.expected == nil {
+				assert.Nil(t, got)
+			} else {
+				assert.NotNil(t, got)
+				assert.Equal(t, *tt.expected, *got)
+			}
+		})
+	}
+}
+
+// TestDerefStr 测试 DerefStr 函数
+func TestDerefStr(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    *string
+		expected string
+	}{
+		{
+			name:     "nil pointer",
+			input:    nil,
+			expected: "",
+		},
+		{
+			name:     "non-nil pointer",
+			input:    strPtr("test@example.com"),
+			expected: "test@example.com",
+		},
+		{
+			name:     "empty string pointer",
+			input:    strPtr(""),
+			expected: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := DerefStr(tt.input)
+			assert.Equal(t, tt.expected, got)
+		})
+	}
+}
+
+// strPtr 辅助函数，返回字符串指针
+func strPtr(s string) *string {
+	return &s
+}
+
+// TestGender_Constants 测试性别常量
+func TestGender_Constants(t *testing.T) {
+	assert.Equal(t, Gender(0), GenderUnknown)
+	assert.Equal(t, Gender(1), GenderMale)
+	assert.Equal(t, Gender(2), GenderFemale)
+}
+
+// TestUserStatus_Constants 测试用户状态常量
+func TestUserStatus_Constants(t *testing.T) {
+	assert.Equal(t, UserStatus(0), UserStatusInactive)
+	assert.Equal(t, UserStatus(1), UserStatusActive)
+	assert.Equal(t, UserStatus(2), UserStatusLocked)
+	assert.Equal(t, UserStatus(3), UserStatusDisabled)
+}
+
+// TestUser_TableName 测试用户表名
+func TestUser_TableName(t *testing.T) {
+	user := User{}
+	assert.Equal(t, "users", user.TableName())
+}
+
+// TestUser_DefaultValues 测试用户默认值
+func TestUser_DefaultValues(t *testing.T) {
+	user := User{}
+	assert.Equal(t, GenderUnknown, user.Gender)
+	assert.Equal(t, UserStatusInactive, user.Status)
+	assert.False(t, user.TOTPEnabled)
+}
+
+// TestStrPtr_DerefStr_RoundTrip 测试往返
+func TestStrPtr_DerefStr_RoundTrip(t *testing.T) {
+	original := "test@example.com"
+	ptr := StrPtr(original)
+	got := DerefStr(ptr)
+	assert.Equal(t, original, got)
+
+	// 注意：StrPtr("") 返回 nil，不是指向空字符串的指针
+	// 这是设计决定的，空字符串表示该字段未设置
+}
+
+// TestStrPtr_NilDeref 测试 nil 解引用
+func TestStrPtr_NilDeref(t *testing.T) {
+	// 空字符串返回 nil
+	assert.Nil(t, StrPtr(""))
+	// nil 解引用返回空字符串
+	assert.Equal(t, "", DerefStr(nil))
+}