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

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

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Co-authored-by: Piotr Wilkin (ilintar) <redacted>