Support ChatGPT-branded Desktop app builds (#33901)

## Why

The Desktop app can use Codex or ChatGPT branding while retaining stable platform identities. CLI discovery and TUI handoff should not depend on a single display name or hardcoded executable path.

## What changed

- On macOS, search for both `ChatGPT.app` and `Codex.app`, and accept only bundles with the `com.openai.codex` identifier.
- On Windows, detect installs by their package app ID and resolve the `codex` protocol executable from the AppX manifest before handing off a TUI session.
- Use “Desktop app” consistently in CLI and TUI user-facing text.

## Testing

Add coverage for selecting a ChatGPT-named Codex bundle and rejecting the classic ChatGPT bundle.

GitOrigin-RevId: b84a56eb6e960712152e1a9b023ab530ec9a354a

codex-rs/tui/src/slash_command.rs

@@ -93,7 +93,7 @@ impl SlashCommand {
             SlashCommand::Delete => "permanently delete this session and exit",
             SlashCommand::Clear => "clear the terminal and start a new chat",
             SlashCommand::Fork => "fork the current chat",
-            SlashCommand::App => "continue this session in Codex Desktop",
+            SlashCommand::App => "continue this session in the Desktop app",
             SlashCommand::Quit | SlashCommand::Exit => "exit Codex",
             SlashCommand::Copy => "copy last response as markdown",
             SlashCommand::Raw => "toggle raw scrollback mode for copy-friendly terminal selection",