Fix #7951: add cancellation checks to polynomial gcd_prs and HNF computation

Add checkpoint() call in gcd_prs() main loop so polynomial GCD
computation respects rlimit/timeout. Add cancellation callback to
HNF calculation so it can be interrupted when the solver is cancelled.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/math/polynomial/polynomial.cpp

@@ -3922,6 +3922,7 @@ namespace polynomial {
 
             unsigned counter = 0;
             while (true) {
+                checkpoint();
                 (void)counter;
                 SASSERT(degree(pp_u, x) >= degree(pp_v, x));
                 unsigned delta = degree(pp_u, x) - degree(pp_v, x);