Refactor and modernize JSON converter test code

Refactor test code in Jellyfin.Extensions.Tests for clarity and consistency:
- Use explicit object initializers and collection expressions
- Standardize field naming and use of this.
- Add/improve XML doc comments for test methods
- Use new(...) syntax for Guid/Version instantiation
- Convert file-scoped to block-scoped namespaces in key tests
- Nest test classes and use instance methods in enum tests
- Enable XML docs and suppress select warnings in csproj
- No changes to test or converter logic; style and maintainability only

src/Jellyfin.CodeAnalysis/obj/Debug/netstandard2.0/Jellyfin.CodeAnalysis.AssemblyInfo.cs

@@ -14,7 +14,7 @@ using System.Reflection;
 [assembly: System.Reflection.AssemblyCompanyAttribute("Jellyfin.CodeAnalysis")]
 [assembly: System.Reflection.AssemblyConfigurationAttribute("Debug")]
 [assembly: System.Reflection.AssemblyFileVersionAttribute("1.0.0.0")]
-[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+48569427a5cba735184e017148b7c71f665279bc")]
+[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+d5522c6fb3c7affc827cf2b509dadab833a237a4")]
 [assembly: System.Reflection.AssemblyProductAttribute("Jellyfin.CodeAnalysis")]
 [assembly: System.Reflection.AssemblyTitleAttribute("Jellyfin.CodeAnalysis")]
 [assembly: System.Reflection.AssemblyVersionAttribute("1.0.0.0")]

src/Jellyfin.CodeAnalysis/obj/Debug/netstandard2.0/Jellyfin.CodeAnalysis.AssemblyInfoInputs.cache

@@ -1 +1 @@
-5afb18d45c1a1f35abd23adbc2ff716c2f51ec4673ea08b8765ec2e818a59a28
+9a6eadffef3683b1209238dc99619e9ba917f9a0f07b616fdd1f9e8cd1c88579