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
