server : use common_token_to_piece instead of common_detokenize (#11740)

server : use common_token_to_piece instead of common_detokenize (#11740)

* server : use common_token_to_piece instead of common_detokenize

This commit replaces the call to common_detokenize with
common_token_to_piece in the populate_token_probs.

The motivation for this change is to avoid an issue where
common_detokenize would remove the word boundary character for tokens,
which caused a regression in the server generated token probabilities.

Resolves: https://github.com/ggerganov/llama.cpp/issues/11728

* squash! server : use common_token_to_piece instead of common_detokenize

Use common_token_to_piece for post_sampling_probs as well.