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

tests/Jellyfin.Extensions.Tests/Json/Converters/JsonBoolNumberTests.cs

@@ -62,6 +62,8 @@ namespace Jellyfin.Extensions.Tests.Json.Converters
         /// <returns>A property test result.</returns>
         [Property]
         public Property Deserialize_NonZeroInt_True(NonZeroInt input)
-            => JsonSerializer.Deserialize<bool>(input.ToString(), this._jsonOptions).ToProperty();
+        {
+            return JsonSerializer.Deserialize<bool>(input.ToString(), this._jsonOptions).ToProperty();
+        }
     }
 }