server: fix passing prompt as tokens

[z80maniac]

This PR fixes a crash in server that happens if prompt is an array of tokens.

Before:

❯ curl -sS --data '{"prompt": [15043,29892,590,1024,338], "n_predict": 4}' http://127.0.0.1:8080/completion | jq .content
curl: (52) Empty reply from server

terminate called after throwing an instance of 'nlohmann::json_abi_v3_11_2::detail::type_error'
  what():  [json.exception.type_error.302] type must be string, but is array
fish: Job 1, './server -m /opt/models/text/ll…' terminated by signal SIGABRT (Abort)

After:

❯ curl -sS --data '{"prompt": [15043,29892,590,1024,338], "n_predict": 4}' http://127.0.0.1:8080/completion | jq .content
" John and I have"

[phymbert]

it would be nice to add a test on that feature. Else it will probably disappear again in the future