Fix mutation of input iterators in seq_arr_erase_it

The seq_arr_erase_it function was modifying start_it during the free_func loop,
which led to incorrectly apply the free_func on the whole array.

This commit makes sure the elements "freed" are only those in the range and not the whole seq_arr.

* Fixes the logic by introducing a temporary iterator for the free loop,
  preserving start_it and end_it for correct offset calculations.

* Adds a regression test in test_seq_arr to verify that only the selected
  subrange is erased and that free_func is called exactly on that range.

common/utils/ds/seq_arr.c

@@ -123,11 +123,10 @@ void seq_arr_erase_it(seq_arr_t* arr, void* start_it, void* end_it, void (*free_
     return;
 
   if (free_func != NULL) {
-    void* start_it = seq_arr_front(arr);
-    void* end_it = seq_arr_end(arr);
-    while (start_it != end_it) {
-      free_func(start_it);
-      start_it = seq_arr_next(arr, start_it);
+    void* it = start_it;
+    while (it != end_it) {
+      free_func(it);
+      it = seq_arr_next(arr, it);
     }
   }