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This allows building:
-  only `mistral.rs` engine: `--no-default-features --features mistralrs`  
- or only `llama.cpp` engine: `--no-default-features --features llamacpp`. 

Since llama.cpp became a default we'd only tested building both at once. The docs already said we supported that but there was some combo of Rust features that didn't build. This is the fix.