Function prepare_result 

pub unsafe fn prepare_result(
    data: Value,
    result_buf: *mut *mut u8,
    result_len: *mut usize,
)

Serialize a Value and hand the buffer to the framework.

Uses into_boxed_slice() so len == capacity is guaranteed — no reliance on shrink_to_fit (which is only a hint).

Safety

Same requirements as return_success.