test: 添加 TimerHelper 时区功能测试用例

- 验证默认时区为 UTC
- 测试 SetTimeZone 更新当前时区功能
- 确保 GetTodayStart、GetWeekStart、GetMonthStart、GetYearStart 系列函数遵循时区设置
- 验证 Unix 时间戳的绝对性（不受时区影响）
- 测试空值和无效时区 ID 的默认回退行为

Author: AlianBlank

GameFrameX.Foundation.Tests/Utility/TimerHelperTimeZoneTests.cs

@@ -0,0 +1,200 @@
+using System;
+using GameFrameX.Foundation.Utility;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace GameFrameX.Foundation.Tests.Utility
+{
+    /// <summary>
+    /// TimerHelper 时区功能测试
+    /// </summary>
+    public class TimerHelperTimeZoneTests : IDisposable
+    {
+        private readonly ITestOutputHelper _output;
+
+        /// <summary>
+        /// 构造函数，初始化测试环境
+        /// </summary>
+        /// <param name="output">测试输出帮助类</param>
+        public TimerHelperTimeZoneTests(ITestOutputHelper output)
+        {
+            _output = output;
+            // Ensure default state is UTC before each test
+            TimerHelper.SetTimeZone(TimeZoneInfo.Utc);
+        }
+
+        /// <summary>
+        /// 释放资源，重置测试环境
+        /// </summary>
+        public void Dispose()
+        {
+            // Reset to UTC after each test to avoid side effects
+            TimerHelper.SetTimeZone(TimeZoneInfo.Utc);
+        }
+
+        /// <summary>
+        /// 测试默认时区应为 UTC
+        /// </summary>
+        [Fact]
+        public void DefaultTimeZone_ShouldBeUtc()
+        {
+            Assert.Equal(TimeZoneInfo.Utc.Id, TimerHelper.CurrentTimeZone.Id);
+            
+            var now = TimerHelper.GetNow();
+            var utcNow = DateTime.UtcNow;
+            
+            // Allow small difference for execution time
+            Assert.True((now - utcNow).Duration().TotalSeconds < 1);
+        }
+
+        /// <summary>
+        /// 测试 SetTimeZone 应正确更新当前时区
+        /// </summary>
+        [Fact]
+        public void SetTimeZone_ShouldUpdateCurrentTimeZone()
+        {
+            // Use a fixed offset timezone for reliable testing
+            var offset = TimeSpan.FromHours(8);
+            var customTimeZone = TimeZoneInfo.CreateCustomTimeZone("Test+8", offset, "Test+8", "Test+8");
+
+            TimerHelper.SetTimeZone(customTimeZone);
+            Assert.Equal(customTimeZone.Id, TimerHelper.CurrentTimeZone.Id);
+
+            var now = TimerHelper.GetNow();
+            var utcNow = DateTime.UtcNow;
+            
+            // Expected: Now = UtcNow + 8 hours
+            var expectedNow = utcNow + offset;
+            Assert.True((now - expectedNow).Duration().TotalSeconds < 1);
+        }
+
+        /// <summary>
+        /// 测试 GetTodayStart 系列函数应遵循时区设置
+        /// </summary>
+        [Fact]
+        public void GetTodayStart_ShouldRespectTimeZone()
+        {
+            var offset = TimeSpan.FromHours(8);
+            var customTimeZone = TimeZoneInfo.CreateCustomTimeZone("Test+8", offset, "Test+8", "Test+8");
+            TimerHelper.SetTimeZone(customTimeZone);
+
+            var now = TimerHelper.GetNow();
+            var todayStart = TimerHelper.GetTodayStartTime();
+            var todayStartTimestamp = TimerHelper.GetTodayStartTimestamp();
+
+            // Verify DateTime parts
+            Assert.Equal(now.Date, todayStart);
+            Assert.Equal(0, todayStart.Hour);
+
+            // Verify Timestamp
+            // Construct expected DateTimeOffset
+            var expectedTime = new DateTimeOffset(todayStart, offset);
+            Assert.Equal(expectedTime.ToUnixTimeSeconds(), todayStartTimestamp);
+        }
+
+        /// <summary>
+        /// 测试 GetWeekStart 系列函数应遵循时区设置
+        /// </summary>
+        [Fact]
+        public void GetWeekStart_ShouldRespectTimeZone()
+        {
+            var offset = TimeSpan.FromHours(8);
+            var customTimeZone = TimeZoneInfo.CreateCustomTimeZone("Test+8", offset, "Test+8", "Test+8");
+            TimerHelper.SetTimeZone(customTimeZone);
+
+            var now = TimerHelper.GetNow();
+            var weekStart = TimerHelper.GetWeekStartTime();
+            var weekStartTimestamp = TimerHelper.GetWeekStartTimestamp();
+
+            // Verify it is indeed the start of the week (Monday)
+            Assert.Equal(DayOfWeek.Monday, weekStart.DayOfWeek);
+            Assert.True(weekStart <= now);
+            Assert.True((now - weekStart).TotalDays < 7);
+            Assert.Equal(0, weekStart.Hour);
+
+            // Verify Timestamp
+            var expectedTime = new DateTimeOffset(weekStart, offset);
+            Assert.Equal(expectedTime.ToUnixTimeSeconds(), weekStartTimestamp);
+        }
+
+        /// <summary>
+        /// 测试 GetMonthStart 系列函数应遵循时区设置
+        /// </summary>
+        [Fact]
+        public void GetMonthStart_ShouldRespectTimeZone()
+        {
+            var offset = TimeSpan.FromHours(-5); // Test negative offset
+            var customTimeZone = TimeZoneInfo.CreateCustomTimeZone("Test-5", offset, "Test-5", "Test-5");
+            TimerHelper.SetTimeZone(customTimeZone);
+
+            var now = TimerHelper.GetNow();
+            var monthStart = TimerHelper.GetMonthStartTime();
+            var monthStartTimestamp = TimerHelper.GetMonthStartTimestamp();
+
+            Assert.Equal(now.Year, monthStart.Year);
+            Assert.Equal(now.Month, monthStart.Month);
+            Assert.Equal(1, monthStart.Day);
+            Assert.Equal(0, monthStart.Hour);
+
+            // Verify Timestamp
+            var expectedTime = new DateTimeOffset(monthStart, offset);
+            Assert.Equal(expectedTime.ToUnixTimeSeconds(), monthStartTimestamp);
+        }
+
+        /// <summary>
+        /// 测试 GetYearStart 系列函数应遵循时区设置
+        /// </summary>
+        [Fact]
+        public void GetYearStart_ShouldRespectTimeZone()
+        {
+            var offset = TimeSpan.FromHours(9);
+            var customTimeZone = TimeZoneInfo.CreateCustomTimeZone("Test+9", offset, "Test+9", "Test+9");
+            TimerHelper.SetTimeZone(customTimeZone);
+
+            var now = TimerHelper.GetNow();
+            var yearStart = TimerHelper.GetYearStartTime();
+            var yearStartTimestamp = TimerHelper.GetYearStartTimestamp();
+
+            Assert.Equal(now.Year, yearStart.Year);
+            Assert.Equal(1, yearStart.Month);
+            Assert.Equal(1, yearStart.Day);
+            Assert.Equal(0, yearStart.Hour);
+
+            // Verify Timestamp
+            var expectedTime = new DateTimeOffset(yearStart, offset);
+            Assert.Equal(expectedTime.ToUnixTimeSeconds(), yearStartTimestamp);
+        }
+
+        /// <summary>
+        /// 测试 Unix 时间戳应保持绝对性（不受时区设置影响，始终基于 UTC）
+        /// </summary>
+        [Fact]
+        public void UnixTimestamp_ShouldBeAbsolute()
+        {
+            // Timestamp should be the same regardless of timezone setting because it represents absolute time
+            
+            TimerHelper.SetTimeZone(TimeZoneInfo.Utc);
+            var ts1 = TimerHelper.UnixTimeSeconds();
+
+            var customTimeZone = TimeZoneInfo.CreateCustomTimeZone("Test+10", TimeSpan.FromHours(10), "Test+10", "Test+10");
+            TimerHelper.SetTimeZone(customTimeZone);
+            var ts2 = TimerHelper.UnixTimeSeconds();
+
+            // Allow small difference for execution time
+            Assert.True(Math.Abs(ts2 - ts1) < 2);
+        }
+        [Fact]
+        public void SetTimeZone_WithNull_ShouldDefaultToUtc()
+        {
+            TimerHelper.SetTimeZone((TimeZoneInfo)null);
+            Assert.Equal(TimeZoneInfo.Utc.Id, TimerHelper.CurrentTimeZone.Id);
+        }
+
+        [Fact]
+        public void SetTimeZone_WithInvalidId_ShouldDefaultToUtc()
+        {
+            TimerHelper.SetTimeZone("InvalidTimeZoneId");
+            Assert.Equal(TimeZoneInfo.Utc.Id, TimerHelper.CurrentTimeZone.Id);
+        }
+    }
+}