Fix soundess issue for bags with negative multiplicity (#7539)

Fix soundess issue for bags with negative multiplicity (#7539)

This PR fixes soundness issues found by cvc5 fuzzy sygus when it answers unsat for sat problems.
One error was with the rewrite rule (bag.count x (bag x c) = c which did not account for cases when c is negative.
This conflicts with fact that all multiplicities are non-negative.
Another error was that the difference_remove inference rule didn't consider the negative case for (count e B)

(= (count e (difference_remove A B)) (ite (=  (count e B) 0) (count e A) 0))) ; not enough
(= (count e (difference_remove A B)) (ite (<= (count e B) 0) (count e A) 0))) ; the right rule