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/JsonBoolStringTests.cs

@@ -23,7 +23,7 @@ namespace Jellyfin.Extensions.Tests.Json.Converters
         [InlineData(@"{ ""Value"": ""false"" }", false)]
         public void Deserialize_String_Valid_Success(string input, bool output)
         {
-            TestStruct s = JsonSerializer.Deserialize<TestStruct>(input, _jsonOptions);
+            TestStruct s = JsonSerializer.Deserialize<TestStruct>(input, this._jsonOptions);
             Assert.Equal(s.Value, output);
         }
 
@@ -32,7 +32,7 @@ namespace Jellyfin.Extensions.Tests.Json.Converters
         [InlineData(false, "false")]
         public void Serialize_Bool_Success(bool input, string output)
         {
-            string value = JsonSerializer.Serialize(input, _jsonOptions);
+            string value = JsonSerializer.Serialize(input, this._jsonOptions);
             Assert.Equal(value, output);
         }