model : use exact GELU for ModernBERT encoders (#30108)
* model : use exact GELU for ModernBERT encoders
Assisted-by: Codex
* model : keep tanh GELU aliases on ggml_geglu
Assisted-by: Claude Opus 5.5
* model : map gelu_python to ggml_geglu_erf
Assisted-by: Claude Opus 5.5