detect Bun installs in CLI update banner (#5074)

## Summary
- detect Bun-managed installs in the JavaScript launcher and set a
dedicated environment flag
- show a Bun-specific upgrade command in the update banner when that
flag is present

Fixes #5012

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https://chatgpt.com/codex/tasks/task_i_68e95c439494832c835bdf34b3b1774e

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Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/tui/src/lib.rs

@@ -310,6 +310,7 @@ async fn run_ratatui_app(
 
         let current_version = env!("CARGO_PKG_VERSION");
         let exe = std::env::current_exe()?;
+        let managed_by_bun = std::env::var_os("CODEX_MANAGED_BY_BUN").is_some();
         let managed_by_npm = std::env::var_os("CODEX_MANAGED_BY_NPM").is_some();
 
         let mut content_lines: Vec<Line<'static>> = vec![
@@ -330,7 +331,14 @@ async fn run_ratatui_app(
             Line::from(""),
         ];
 
-        if managed_by_npm {
+        if managed_by_bun {
+            let bun_cmd = "bun install -g @openai/codex@latest";
+            content_lines.push(Line::from(vec![
+                "Run ".into(),
+                bun_cmd.cyan(),
+                " to update.".into(),
+            ]));
+        } else if managed_by_npm {
             let npm_cmd = "npm install -g @openai/codex@latest";
             content_lines.push(Line::from(vec![
                 "Run ".into(),