speculative : fix "ngram-map-k4v" name in logging (#24253)
This is a non-functional change.
When using `--spec-type ngram-map-k4v`, the log messages at startup and
runtime say `ngram-map-k`. Added logic in the in the constructor of
`common_speculative_impl_ngram_map_k` to pass the correct
`COMMON_SPECULATIVE_TYPE_NGRAM_MAP_K4V` when `config.key_only` is
`false`.
After this change, the log messages use the correct name.
