[compiler] Update for Zod v3/v4 compatibility

Partial redo of #34710. The changes there tried to use `z.function(args, return)` to be compatible across Zod v3 and v4, but Zod 4's function API has completely changed. Instead, I've updated to just use `z.any()` where we expect a function, and manually validate that it's a function before we call the value. We already have validation of the return type (also using Zod).