Add @Nullable to a functional-interface return type argument fed a null-returning lambda

When a null-returning lambda is passed for a functional-interface parameter (such as Function<R, T>) of a method that consumes the produced value in a null-tolerant way, annotates that interface's return type argument @Nullable (Function<R, @Nullable T>) so returning null through the callback is legal under JSpecify/NullAway. This is distinct from widening a pass-through type-parameter bound (handled separately); a de-overlap guard keeps the two from both firing. Only parameters of recognized functional interfaces, on methods declared in the working set, are changed; already-@Nullable and wildcard/raw arguments are left untouched.