tools : fix uninitialized llama_batch in server (#13436)
* add constructor to initialize server_context::batch, preventing destructor's call to llama_batch_free from causing an invalid free()
* Update tools/server/server.cpp
Co-authored-by: Xuan-Son Nguyen <thichthat@gmail.com>
* use C++11 initializer syntax
* switch from Copy-list-initialization to Direct-list-initialization
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Co-authored-by: Xuan-Son Nguyen <thichthat@gmail.com>
