model : support classifier_pooling for rerankers (#29627)

* model : support classifier_pooling for ModernBERT rerankers

Assisted-by: Claude Opus 5.5

* model : read classifier pooling type in load_hparams

Write classifier.pooling_type from _try_set_pooling_type whenever the
config has classifier_pooling, and read it in
llama_model_base::load_hparams. ModernBERT falls back to mean when it
is unspecified.

Assisted-by: Claude Opus 5.5

* conversion : only accept cls and mean for classifier_pooling

Assisted-by: Claude Opus 5.5

* model : rename classifier_pooling_type to pooling_type_cls

Assisted-by: Claude Opus 5.5