Design, Synthesis, DFT, and Biological Evaluation of Nano‐Sized Pt(II) and Cu(II) Complexes of 2‐(Benzo[d]oxazol‐2‐yl)phenylphosphoramidic Dichloride: Spectral Analysis, Cell Cycle Arrest, Apoptosis Assay, Cytotoxicity, and DNA Binding/Cleavage