SortedSet in C#: .NET 10, MSTest 4.5.2 and new SortedSet tests - #2379

Moves the SortedSetInCSharp sample from net6.0 to net10.0 and updates MSTest to 4.5.2, Microsoft.NET.Test.Sdk to 18.11.0 and coverlet.collector to 10.1.0. Both code files use file-scoped namespaces, every Assert.AreEqual now takes the expected value first, the asserts read the Count property instead of LINQ Count(), the SetEquals test compares against a reordered list with a duplicate instead of the same reference, and TryGetValue reads its out value. New tests cover the Add() return value on a duplicate, a null element in a set of strings, a comparer that drops ties and one that breaks them, Min and Max, the live GetViewBetween() view, and subset checks on two equal sets (22 tests in total).