server: prevent data race from HTTP threads (#18263)

server: prevent data race from HTTP threads (#18263)

* server: prevent data race from HTTP threads

* fix params

* fix default_generation_settings

* nits: make handle_completions_impl looks less strange

* stricter const

* fix GGML_ASSERT(idx < states.size())

* move index to be managed by server_response_reader

* http: make sure req & res lifecycle are tied together

* fix compile

* fix index handling buggy

* fix data race for lora endpoint

* nits: fix shadow variable

* nits: revert redundant changes

* nits: correct naming for json_webui_settings