iserith
Magic Wordsmith
I'm going to call you on this. Show me anything that suggests a random fighter somehow knows that he can suicide his way out of a polymorph spell. Is it in the spell text? Nope. Is it in the fighter write up in the PHB? Nope. Combat description in the PHB? Nope. Go ahead, find something. I dare you. I would say is that crickets I heard, but I think the toad is eating them.
The fighter doesn't have to know that he or she can "suicide" his or her way out of a polymorph spell. That knowledge is not required for the player to describe the character in toad form leaping at the extended blade of a nearby ally. And if the DM for some reason demands an explanation as to why the toad did that, I've already given you one upthread. One of many that could easily be imagined.