Text this: Synthesis and Characterization of (OC‐6‐33)‐Dichloridobis((4‐(13C2)ethoxy)‐4‐(13C4)oxobutanoato)bis((15N)ethylamine)platinum(IV). Time‐Dependent NMR Spectroscopy and HPLC‐MS Investigations on its Stability in Cell Culture Medium