Dictionary key by value: retarget net10.0, add prebuilt reverse map benchmarks

[vladimir-pecanac-main]

Sample update for the article "Fastest Way to Get a Dictionary Key by Value in C#" (https://code-maze.com/csharp-fastest-way-to-get-a-dictionary-key-by-value/). Touches only collections-dictionary/GetDictionaryKeyByValue.

Changes:

• Both projects retargeted from net8.0 to net10.0. ImplicitUsings and Nullable were already enabled.
• New ReverseDictionaryLookup class. It builds a reverse Dictionary<string, string> and a FrozenDictionary<string, string> once, with the same TryAdd reversal UseReverseDictionary() uses, and answers each lookup with a single TryGetValue.
• Two new benchmarks, UsePrebuiltReverseDictionary and UsePrebuiltFrozenDictionary. The maps are built in a [GlobalSetup], so each benchmark times only the lookup. The existing four benchmarks are unchanged: they still charge the two reversing methods the full build on every call, which is the single-lookup case the article's table measures.
• Four new tests, one per new method in each existing test class (DictionaryHelperPositiveTest and DictionaryHelperNullTest), same GivenA...When...Then... naming. 8 tests become 12.
• DictionaryHelper.cs is not touched; the article's four methods stay byte-identical to it.

Packages, re-queried from api.nuget.org on 2026-09-23 (newest stable):

• BenchmarkDotNet 0.13.12 -> 0.15.8 (0.16.0-preview.1 exists and is not taken)
• Microsoft.NET.Test.Sdk 17.8.0 -> 18.10.1 (the batch plan said 18.10.0; the re-query wins)
• xunit 2.5.3 -> 2.9.3
• xunit.runner.visualstudio 2.5.3 -> 4.0.0
• coverlet.collector 6.0.0 -> 10.0.1

Verified locally on SDK 10.0.302, runtime .NET 10.0.10: dotnet build -c Release 0 warnings, 0 errors; dotnet test 12 passed, 0 failed (8 existing + 4 new), in Release and Debug; dotnet list package --vulnerable --include-transitive reports no vulnerable packages on either project.