Mistral explicitly sets `moe` and `llama_4_scaling` to `null` in
params.json, breaking `key in dict` checks during conversion. Replace
with `dict.get(key) is not None` where this matters.
Fixes `convert-hf-to-gguf.py --mistral-format Mistral-Medium-3.5-128B`
* move conversion code to a dedicated conversion directory and split the files akin to the src/models architecture
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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>