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/JsonStringConverterTests.cs

@@ -26,7 +26,7 @@ namespace Jellyfin.Extensions.Tests.Json.Converters
         [InlineData("false", "false")]
         public void Deserialize_String_Valid_Success(string input, string output)
         {
-            var deserialized = JsonSerializer.Deserialize<string>(input, _jsonSerializerOptions);
+            string? deserialized = JsonSerializer.Deserialize<string>(input, this._jsonSerializerOptions);
             Assert.Equal(deserialized, output);
         }
 
@@ -35,7 +35,7 @@ namespace Jellyfin.Extensions.Tests.Json.Converters
         {
             const string? input = "123";
             const int output = 123;
-            var deserialized = JsonSerializer.Deserialize<int>(input, _jsonSerializerOptions);
+            int deserialized = JsonSerializer.Deserialize<int>(input, this._jsonSerializerOptions);
             Assert.Equal(output, deserialized);
         }
     }