fix: correct operator precedence in ForAllFields reverse iteration (#8991)

* fix: correct operator precedence in ForAllFields reverse iteration

The expression `size() - i + 1` evaluates as `(size() - i) + 1` due to
left-to-right associativity, producing an out-of-bounds index when
reverse=true. For a vector of size N, the first iteration (i=0) accesses
index N+1, which is 2 past the last valid index.

Changed to `size() - (i + 1)` to match the correct implementation
already present in bfbs_gen.h:192.

Bug: CWE-125 (Out-of-bounds Read), CWE-783 (Operator Precedence Error)

* test: add ForAllFieldsReverseTest for reverse iteration correctness

Verify that ForAllFields with reverse=true iterates fields in
descending ID order. Tests both Stat (3 fields) and Monster
(many fields with non-sequential definition order) tables.

---------

Co-authored-by: Tulgaa <tulgaa.kek@gmail.com>

src/reflection.cpp

@@ -389,7 +389,7 @@ void ForAllFields(const reflection::Object* object, bool reverse,
 
   for (size_t i = 0; i < field_to_id_map.size(); ++i) {
     func(object->fields()->Get(
-        field_to_id_map[reverse ? field_to_id_map.size() - i + 1 : i]));
+        field_to_id_map[reverse ? field_to_id_map.size() - (i + 1) : i]));
   }
 }