common : read a GGUF's trained context from its metadata
common_get_gguf_n_ctx_train opens only the file's metadata (no_alloc,
like common_get_decision_type) and reads <arch>.context_length, so a
caller can learn the trained context without loading the model. It
accepts both u32 and u64 values and returns 0 when the file is missing,
unreadable, invalid, or reports no context length.
Assisted-by: pi:zai-org/GLM-5.3-Flash
