fix(cpp): const_cast map keys during lowering for ownership transfer

std::map keys are const, but the ABI lowering needs to call leak() on
string keys to transfer ownership to the flat buffer. Use const_cast
since the map is consumed during the lowering operation.

Author: yordis

crates/cpp/src/lib.rs

@@ -3560,6 +3560,8 @@ impl<'a, 'b> Bindgen for FunctionBindgen<'a, 'b> {
                 let ptr = format!("ptr{tmp}");
                 let len = format!("len{tmp}");
                 let idx = format!("idx{tmp}");
+                let entry_name = format!("entry{tmp}");
+                let key_type = self.r#gen.type_name(key, &self.namespace, Flavor::InStruct);
                 let entry = self.r#gen.sizes.record([*key, *value]);
                 let size = entry.size.format(POINTER_SIZE_EXPRESSION);
                 let align = entry.align.format(POINTER_SIZE_EXPRESSION);
@@ -3571,10 +3573,12 @@ impl<'a, 'b> Bindgen for FunctionBindgen<'a, 'b> {
                     ptr_type = self.r#gen.r#gen.opts.ptr_type()
                 );
                 uwriteln!(self.src, "size_t {idx} = 0;");
+                uwriteln!(self.src, "for (auto& {entry_name} : {val}) {{");
                 uwriteln!(
                     self.src,
-                    "for (auto&& [iter_map_key, iter_map_value] : {val}) {{"
+                    "auto& iter_map_key = const_cast<{key_type}&>({entry_name}.first);"
                 );
+                uwriteln!(self.src, "auto& iter_map_value = {entry_name}.second;");
                 uwriteln!(self.src, "auto base = {ptr} + {idx} * {size};");
                 uwrite!(self.src, "{}", body.0);
                 uwriteln!(self.src, "++{idx};");