fix: gemma 4 template (#21326)

common/chat-peg-parser.h

@@ -17,7 +17,9 @@ class common_chat_peg_mapper {
 
     virtual void from_ast(const common_peg_ast_arena & arena, const common_peg_parse_result & result);
     virtual void map(const common_peg_ast_node & node);
-    private:
+  protected:
+    virtual std::string normalize_container_value(const std::string & input);
+  private:
       // Tool call handling state
       std::optional<common_chat_tool_call> pending_tool_call;  // Tool call waiting for name
       common_chat_tool_call *              current_tool          = nullptr;
@@ -30,6 +32,13 @@ class common_chat_peg_mapper {
       std::string & args_target();
 };
 
+class common_chat_peg_gemma4_mapper : public common_chat_peg_mapper {
+  public:
+    common_chat_peg_gemma4_mapper(common_chat_msg & msg) : common_chat_peg_mapper(msg) {}
+  protected:
+    std::string normalize_container_value(const std::string & input) override;
+};
+
 struct content_structure;
 struct tool_call_structure;