conversion: accept BitNetForCausalLM architecture name (#25769)

Microsoft BitNet Hugging Face configs use BitNetForCausalLM while the
converter only registered BitnetForCausalLM, causing conversion to fail
with "Model BitNetForCausalLM is not supported".

Register both spellings in TEXT_MODEL_MAP and the Bitnet model class.

Fixes ggml-org/llama.cpp#25629

conversion/__init__.py

@@ -31,6 +31,7 @@ TEXT_MODEL_MAP: dict[str, str] = {
     "BertForSequenceClassification": "bert",
     "BertModel": "bert",
     "BitnetForCausalLM": "bitnet",
+    "BitNetForCausalLM": "bitnet",
     "BloomForCausalLM": "bloom",
     "BloomModel": "bloom",
     "CamembertModel": "bert",