Refactor structure, add tests, and improve cache API

- Move main library to AspNetCoreCacheKit folder; add AspNetCoreCacheKit.Tests project with xUnit tests
- Make Set methods generic for type safety in CacheService and ICacheService
- Move NullCacheEntry to its own file and make it internal
- Fix key composition logic and improve argument validation
- Update DI extension and project packaging metadata
- Add test helper and comprehensive unit tests for all cache scenarios

Author: simonemarano1993

AspNetCoreCacheKit.Tests/AspNetCoreCacheKit.Tests.csproj

@@ -0,0 +1,25 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.2" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\AspNetCoreCacheKit\AspNetCoreCacheKit.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+</Project>

AspNetCoreCacheKit.Tests/Helpers/CacheServiceFactory.cs

@@ -0,0 +1,31 @@
+using AspNetCoreCacheKit.Models;
+using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Extensions.Options;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace AspNetCoreCacheKit.Tests.Helpers
+{
+    internal static class CacheServiceFactory
+    {
+        public static CacheService Create(
+            bool isEnabled = true,
+            int durationMinutes = 60,
+            Dictionary<string, int>? groupDurations = null)
+        {
+            var memoryCache = new MemoryCache(new MemoryCacheOptions());
+
+            var options = Options.Create(new CacheOptions
+            {
+                IsEnabled = isEnabled,
+                DurationMinutes = durationMinutes,
+                GroupDurations = groupDurations ?? []
+            });
+
+            return new CacheService(memoryCache, options);
+        }
+    }
+}