Build a fresh Command invoking robonix-codegen either directly
(preferred โ picks up $ROBONIX_CODEGEN_BIN / installed bin / target/
in that order) or via cargo run -p robonix-codegen as a last
resort. Caller appends the actual --lang โฆ -I โฆ -o โฆ args.
Probe the active python3 for grpc_tools.protoc. Bails with a single,
copy-pasteable install instruction if either is missing โ this is the
only Python dep rbnx codegen reaches for, so making it explicit up
front is the entire UX cost of not vendoring protoc + grpc_python_plugin.