test:WIP add tests. Change decade context end date.

tests/Test.Ursa/DateTimePickerTests/DateTimeHelperTests.cs

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+using Ursa.Controls;
+
+namespace Test.Ursa.DateTimePicker;
+
+public class DateTimeHelperTests
+{
+    [Fact]
+    public void GetFirstDayOfMonth_ReturnsFirstDay()
+    {
+        var date = new DateTime(2023, 5, 15);
+        var firstDay = date.GetFirstDayOfMonth();
+        Assert.Equal(new DateTime(2023, 5, 1), firstDay);
+    }
+
+    [Fact]
+    public void GetLastDayOfMonth_ReturnsLastDay()
+    {
+        var date = new DateTime(2023, 5, 15);
+        var lastDay = date.GetLastDayOfMonth();
+        Assert.Equal(new DateTime(2023, 5, 31), lastDay);
+    }
+
+    [Fact]
+    public void CompareYearMonth_SameYearMonth_ReturnsZero()
+    {
+        var date1 = new DateTime(2023, 5, 15);
+        var date2 = new DateTime(2023, 5, 20);
+        var result = DateTimeHelper.CompareYearMonth(date1, date2);
+        Assert.Equal(0, result);
+    }
+
+    [Fact]
+    public void CompareYearMonth_DifferentYearMonth_ReturnsDifference()
+    {
+        var date1 = new DateTime(2023, 5, 15);
+        var date2 = new DateTime(2024, 6, 20);
+        var result = DateTimeHelper.CompareYearMonth(date1, date2);
+        Assert.Equal(-13, result);
+    }
+
+    [Fact]
+    public void Min_ReturnsMinimumDate()
+    {
+        var date1 = new DateTime(2023, 5, 15);
+        var date2 = new DateTime(2024, 6, 20);
+        var result = DateTimeHelper.Min(date1, date2);
+        Assert.Equal(date1, result);
+    }
+
+    [Fact]
+    public void Max_ReturnsMaximumDate()
+    {
+        var date1 = new DateTime(2023, 5, 15);
+        var date2 = new DateTime(2024, 6, 20);
+        var result = DateTimeHelper.Max(date1, date2);
+        Assert.Equal(date2, result);
+    }
+
+    [Fact]
+    public void GetDecadeViewRangeByYear_ReturnsCorrectRange()
+    {
+        var year = 2023;
+        var result = DateTimeHelper.GetDecadeViewRangeByYear(year);
+        Assert.Equal((2020, 2029), result);
+    }
+
+    [Fact]
+    public void GetCenturyViewRangeByYear_ReturnsCorrectRange()
+    {
+        var year = 2023;
+        var result = DateTimeHelper.GetCenturyViewRangeByYear(year);
+        Assert.Equal((2000, 2100), result);
+    }
+}