fix(core): fix iterator in backspace handling

This change replaces the reverse iterator loop that holds a stale iterator
across `pop_back()` calls with a pattern that directly accesses
`context.back()` on each iteration. This avoids undefined behavior from
iterator invalidation when mutating the list or vector.

While so far the previous code didn't show problems, it might still access
released memory depending on the implementation. The documentation for
`pop_back()` says "References and iterators to the erased element are
invalidated", so the previous implementation was clearly wrong.

Test-bot: skip

Author: ermshiperete

core/src/ldml/ldml_processor.cpp

@@ -233,15 +233,15 @@ void ldml_event_state::emit_backspace() {
   // Find out what the last actual character was and remove it.
   // attempt to get the last char
   // TODO-LDML: emoji backspace
-  auto end = state->context().rbegin();
-  while (end != state->context().rend()) {
-    if (end->type == KM_CORE_CT_CHAR) {
+  while (!state->context().empty()) {
+    auto end = state->context().back();
+    if (end.type == KM_CORE_CT_CHAR) {
       actions.code_points_to_delete++;
       state->context().pop_back();
       return;
     }
     // else loop again
-    assert(end->type != KM_CORE_CT_END);  // inappropriate here.
+    assert(end.type != KM_CORE_CT_END);  // inappropriate here.
     state->context().pop_back();
  }
   /*

core/tests/unit/ldml/ldml.cpp

@@ -157,21 +157,20 @@ apply_action(
       matched_text = true; // no text to match as context is empty.
     }
     // now, we need to simulate what ldml_processor::emit_backspace() is going to do.
-      auto end = context.rbegin();
-      while (end != context.rend()) {
-        if (end->type == KM_CORE_CT_CHAR) {
-          test_assert(!matched_text);
-          test_assert_equal(end->character, ch); // expect popped char to be same as what's in context
-          matched_text = true;
-          context.pop_back();
-          break;  // exit on first real char
-        }
-        test_assert(end->type != KM_CORE_CT_END);  // inappropriate here.
+    while (!context.empty()) {
+      if (context.back().type == KM_CORE_CT_CHAR) {
+        test_assert(!matched_text);
+        test_assert_equal(context.back().character, ch); // expect popped char to be same as what's in context
+        matched_text = true;
         context.pop_back();
+        break;  // exit on first real char
       }
-      test_assert(matched_text);
+      test_assert(context.back().type != KM_CORE_CT_END);  // inappropriate here.
+      context.pop_back();
     }
+    test_assert(matched_text);
     break;
+  }
   case KM_CORE_IT_PERSIST_OPT:
     std::cout << "  + TODO-LDML: persist_opt()" << std::endl;
     break;