server : refactor middleware and /health endpoint (#9056)

* server : refactor middleware and /health endpoint

* move "fail_on_no_slot" to /slots

* Update examples/server/server.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

* fix server tests

* fix CI

* update server docs

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>