RequestBodyParser does not support scalar JSON values (bool, int, float, string)

What steps will reproduce the problem?

1. Use the Yiisoft\Request\Body\Parser\JsonParser to parse a JSON body containing scalar values such as true, 42, 3.14, or "hello".
2. Observe that the Yiisoft\Request\Body\Parser\JsonParser does not handle these scalar values correctly.

What is the expected result?

The RequestBodyParser should correctly parse JSON bodies containing scalar values (boolean, integer, float, string) and return these values without errors.

json_decode('true');   // Expected result: true (boolean)
json_decode('42');     // Expected result: 42 (integer)
json_decode('3.14');   // Expected result: 3.14 (float)
var_dump(json_decode('-1e3')); // Expected -1000 (float)
json_decode('"hello"');// Expected result: "hello" (string)

What do you get instead?

The RequestBodyParser fails to handle JSON bodies with scalar values and does not return the expected result. Instead, it only supports objects and arrays.

Start with the Yiisoft\Request\Body\Parser\JsonParser entry point and inspect how it currently handles JSON objects and arrays. Verify behavior for true, 42, 3.14, -1e3, and "hello"; done means each scalar value is returned correctly without errors.