Function generate_fish_env_file_content 

pub fn generate_fish_env_file_content(
    hip_visible_devices: &str,
    rocm_path: &str,
    rocm_version: &str,
    rocm_channel: &str,
    gpu_arch: &str,
    hsa_override_gfx_version: &str,
    python_bin: &str,
) -> String

Generate a fish-compatible environment file.

Produces ~/.config/fish/conf.d/mlstack_env.fish that natively loads all ML Stack environment variables without needing to source the bash file. Fish doesn’t support bash syntax (if [ -z ... ], ${VAR:-}, fi, etc.), so this uses idiomatic fish constructs.