TimeSpan in C#: retarget to net10.0, MSTest 4.4.0, readable constructor values, four new tests

[vladimir-pecanac-main]

Updates the dotnet-datetime/TimeSpanInCSharp sample for the article rewrite.

• Both projects retargeted from net7.0 to net10.0.
• Microsoft.NET.Test.Sdk 17.3.2 to 18.10.1, MSTest.TestAdapter and MSTest.TestFramework 2.2.10 to 4.4.0, coverlet.collector 3.1.2 to 10.0.1 (versions re-queried on NuGet on 2026-09-15; 18.10.1 is the newest stable Test.Sdk).
• Divide (both tests), Duration, Multiply and Subtract now build their values with readable arguments (new TimeSpan(4, 0, 0) instead of new TimeSpan(2, 60, 3600)), so the numbers in the article can be checked without arithmetic.
• Two swapped Assert.AreEqual argument pairs fixed (MSTEST0017 in the Duration and Negate tests).
• Four new tests: standard and custom format specifiers, TryParse behaviour, MinValue/MaxValue/Zero range behaviour, and the .NET 9 integer FromSeconds overload.

Build: 0 warnings, 0 errors. Tests: 17/17 passing on .NET 10.0.10, SDK 10.0.302.