Improve guest command handling

[lacraig2]

Summary

• prepend /igloo/utils to guest command PATH in the host client
• move long-command warning into the guesthopper daemon so it appears on /dev/ttyS0
• warn only once per guesthopper daemon process
• harden Python client socket discovery, protocol validation, JSON decoding, and CLI error handling
• run the Python client unittest suite in pull request CI
• commit Cargo.lock and use the Docker Hub toolchain image so CI resolves crates compatible with rustc 1.71.1

Testing

• python3 -m unittest discover -s tests
• python3 -m py_compile guest_cmd.py tests/test_guest_cmd.py
• git diff --check
• docker run --rm -v $PWD:/app -w /app rehosting/embedded-toolchains_rust:latest bash -lc 'rustc --version && CARGO_TARGET_DIR=/tmp/guesthopper-target cargo check --locked'