Fix drain_backtrack to pop trail scopes and compile

The drain_backtrack destructor added in d247df72a called
m_backtrack.pop(), which does not exist on ptr_vector (breaking the
build) and, even as pop_back(), would only drop the work item without
popping the backtracking trail scope it owns - reintroducing the
trail-scope leak that #10196 fixed. Use backtrack(), which pops the trail
scope for in_scope items and then removes the item.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 96a14756-2ffe-4cc3-87e7-49fda1b6113a

src/ast/euf/ho_matcher.cpp

@@ -128,7 +128,7 @@ namespace euf {
             drain_backtrack(ho_matcher &m) : m(m) {}
             ~drain_backtrack() {
                 while (!m.m_backtrack.empty()) {
-                    m.m_backtrack.pop();
+                    m.backtrack();
                 }
             }
         };