common/parser : fix out_of_range crash in throw path (#20424 regression) (#20777)

* chat : fix out_of_range crash in throw path (#20424 regression)

#20424 introduced effective_input = generation_prompt + input, but the
throw path uses input.substr(result.end) where result.end is a position
within effective_input. Every thinking model with a non-empty
generation_prompt crashes with std::out_of_range instead of the intended
error message.

Test crashes on unpatched master, passes with fix:

  cmake -B build -DLLAMA_BUILD_TESTS=ON -DLLAMA_BUILD_TOOLS=OFF
  cmake --build build --target test-chat
  ./build/bin/test-chat

* Update test-chat.cpp

* Update test-chat.cpp

* Update test-chat.cpp

---------

Co-authored-by: Piotr Wilkin (ilintar) <piotr.wilkin@syndatis.com>

common/chat.cpp

@@ -1774,7 +1774,7 @@ common_chat_msg common_chat_peg_parse(const common_peg_arena &          src_pars
             return msg;
         }
         throw std::runtime_error(std::string("Failed to parse input at pos ") + std::to_string(result.end) + ": " +
-                                 input.substr(result.end));
+                                 effective_input.substr(result.end));
     }
 
     common_chat_msg msg;