* server : pass id_slot through the Anthropic /v1/messages conversion
server_chat_convert_anthropic_to_oai copies a small whitelist of common
params from the Anthropic request into the internal OAI-style body and
left id_slot out. Every other completion endpoint honours id_slot; a
proxy that pins a session to the slot holding its KV cache needs the
same on /v1/messages, or it silently falls back to automatic slot
selection and, on hybrid/recurrent models, pays a full re-prefill.
Adds a test asserting a request with id_slot set is served by that
slot, observed via GET /slots.
* server : shorten comments
* rm test
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Co-authored-by: Xuan Son Nguyen <son@huggingface.co>