load_model() decided gguf-vs-HF purely from the literal string 'gguf' in
the model name. A gguf whose alias carries only the quant suffix (e.g.
'coe-gemma4-coding-hc-14b-a4b-q4_k_m', no literal 'gguf') was mis-routed
to the HF/transformers backend, which then failed with "is not a valid
model identifier" (503). Fall back to _resolve_local_gguf(): if the alias
maps to an actual local .gguf, treat it as gguf and route to llama.cpp.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>