Add codex update command (#19933)

## Why

Addresses #9274

Running `codex update` currently starts an interactive Codex session
with `update` as the prompt. That is a rough edge for users who expect a
direct self-update command after seeing the existing update notice, and
it forces them to copy the suggested package-manager command manually.

## What changed

- Added a top-level `codex update` subcommand.
- Reused the existing install-channel detection and update command
runner that the TUI already uses for update prompts.
- Exposed the update-action lookup from `codex-tui` so the CLI can
invoke the same behavior.
- Added CLI coverage to ensure `codex update` is parsed as a subcommand
instead of becoming an interactive prompt.

## Verification

- `cargo test -p codex-cli`
- `cargo test -p codex-tui update_action::tests`

codex-rs/tui/src/lib.rs

@@ -171,6 +171,8 @@ mod tui;
 mod ui_consts;
 pub(crate) mod update_action;
 pub use update_action::UpdateAction;
+#[cfg(not(debug_assertions))]
+pub use update_action::get_update_action;
 mod update_prompt;
 #[cfg(any(not(debug_assertions), test))]
 mod update_versions;

codex-rs/tui/src/update_action.rs

@@ -59,7 +59,7 @@ impl UpdateAction {
 }
 
 #[cfg(not(debug_assertions))]
-pub(crate) fn get_update_action() -> Option<UpdateAction> {
+pub fn get_update_action() -> Option<UpdateAction> {
     UpdateAction::from_install_context(InstallContext::current())
 }