ConcurrentDictionary in C#: retarget net10.0, NUnit 4.6.1, add the unsized naive example

[vladimir-pecanac-main]

Updates the sample for the ConcurrentDictionary in C# article (collections-csharp/ConcurrentDictionary only).

Changes:

• Both projects move from net7.0 to net10.0.
• Test packages, each re-queried on NuGet on 2026-09-23 (newest stable): Microsoft.NET.Test.Sdk 17.3.2 to 18.10.1, NUnit 3.13.3 to 4.6.1, NUnit3TestAdapter 4.2.1 to 6.3.0, NUnit.Analyzers 3.3.0 to 4.15.0, coverlet.collector 3.1.2 to 10.0.1. The batch plan named Microsoft.NET.Test.Sdk 18.10.0; 18.10.1 is current and is what this PR uses.
• NUnit 4.6.1: Assert.IsTrue, Assert.IsFalse and Assert.Greater compile unchanged from NUnit.Framework on net10.0, so no assertion is rewritten and nothing moves to ClassicAssert.
• New NaiveExampleUnsized.cs holds the unsized Dictionary example the article opens with. Program.cs runs it first, inside a try/catch that prints the AggregateException. It has no test because the throw depends on thread timing.
• ContentionExample: the emptyHits local was incremented and never read. It is removed and the empty-state check is called directly, which is the cost the example measures.
• ContentionExampleIntegrationTest: the wall-clock comparison test is renamed to WhenBothVariantsRun_ThenTheSecondVariantTakesMoreTime_Live so the CI filter FullyQualifiedName!~Live skips it. It still runs locally.
• ContentionExampleIntegrationTest and MemoryLeakExampleIntegrationTest: the sut field was created twice, once by its initialiser and again in [SetUp]. The initialiser is dropped.
• Program.cs prints one line before ContentionExample and one before MemoryLeakExample so the program does not look stuck.

Verified locally on SDK 10.0.302, runtime 10.0.10: dotnet build -c Release with 0 warnings and 0 errors; dotnet test with 10 of 10 passing, and 9 of 9 with the CI filter; dotnet list package --vulnerable --include-transitive reports nothing.