An Unusual Dinuclear Ruthenium(III) Complex with a Conjugated Bridging Ligand Derived from Cleavage of a 1,4-Dihydro-1,2,4,5-Tetrazine Ring. Synthesis, Structure, and UV−Vis−NIR Spectroelectrochemical Characterization of a Five-Membered Redox Chain Incorporating Two Mixed-Valence States