jinja: give each for-loop iteration a fresh scope (#2018)

`{% set %}` of a non-loop variable inside a `{% for %}` body leaks
across iterations when the assignment is conditionally skipped. Each
iteration should start with a clean scope, matching standard Jinja2
semantics.

This fixes the issue with GLM-5.2 chat template when:
* turn 1 is a tool call with reasoning
* turn 2 is a tool call without reasoning

In this case, the reasoning content for turn 1 would be wrongly
duplicated to turn 2, resulting in degraded model performance.

common/jinja/runtime.cpp

@@ -596,11 +596,18 @@ value for_statement::execute_impl(context & ctx) {
         loop_obj->insert("length", mk_val<value_int>(filtered_items.size()));
         loop_obj->insert("previtem", i > 0 ? filtered_items[i - 1] : mk_val<value_undefined>("previtem"));
         loop_obj->insert("nextitem", i < filtered_items.size() - 1 ? filtered_items[i + 1] : mk_val<value_undefined>("nextitem"));
-        scope.set_val("loop", loop_obj);
-        scope_update_fns[i](scope);
+        // Use a fresh scope for each iteration so that {% set %} variables
+        // (including ones assigned only conditionally inside the body) do not
+        // leak across iterations. This matches standard Jinja2 semantics, where
+        // each loop iteration starts with a clean scope. State that must
+        // accumulate across iterations has to use namespace(), whose mutations
+        // are applied to the shared object referenced from the enclosing scope.
+        context iter_scope(scope);
+        iter_scope.set_val("loop", loop_obj);
+        scope_update_fns[i](iter_scope);
         try {
             for (auto & stmt : body) {
-                value val = stmt->execute(scope);
+                value val = stmt->execute(iter_scope);
                 result->push_back(val);
             }
         } catch (const continue_statement::signal &) {

tests/test-jinja.cpp

@@ -360,6 +360,18 @@ static void test_loops(testing & t) {
         json::object(),
         "012"
     );
+
+    // {% set %} of a non-loop variable inside a loop body must not leak across
+    // iterations. When a variable is only assigned on some iterations (here via
+    // a conditional branch), the engine must still treat it as undefined on the
+    // iterations where the branch is not taken, rather than reusing the value
+    // from a previous iteration. This matches standard Jinja2 semantics, where
+    // each loop iteration starts with a fresh scope.
+    test_template(t, "conditional set does not leak across iterations",
+        "{%- for m in msgs %}{% if m.x %}{% set r = m.x %}{% endif %}[{{ r|default('-') }}]{% endfor %}",
+        {{"msgs", json::array({json{{"x", "a"}}, json::object(), json{{"x", "c"}}})}},
+        "[a][-][c]"
+    );
 }
 
 static void test_expressions(testing & t) {