Update full-auto description with on-request

[lionelchg]

This PR fixes #6522 by correcting the comment for full-auto in both codex-rs/exec/src/cli.rs and codex-rs/tui/src/cli.rs from -a on-failure to -a on-request to make it coherent with codex-rs/tui/src/lib.rs:97-105:

pub async fn run_main(
    mut cli: Cli,
    codex_linux_sandbox_exe: Option<PathBuf>,
) -> std::io::Result<AppExitInfo> {
    let (sandbox_mode, approval_policy) = if cli.full_auto {
        (
            Some(SandboxMode::WorkspaceWrite),
            Some(AskForApproval::OnRequest),
        )

Running just codex --help or just codex exec --help should now yield the correct description of full-auto CLI argument.

[etraut-openai]

Thanks for the contribution.