Function resolve_binary 

pub fn resolve_binary(
    name: &str,
    env_override: Option<&str>,
) -> Result<PathBuf, RunnerError>

Resolve a binary path by name, optionally using an environment variable override

Resolution order:

1. If env_override is Some, use that value as the path
2. Otherwise, search PATH using which

Errors

Returns RunnerError if the binary cannot be found.