fix: report an appropriate error in the TUI for malformed rules (#9011)

The underlying issue is that when we encountered an error starting a
conversation (any sort of error, though making `$CODEX_HOME/rules` a
file rather than folder was the example in #8803), then we were writing
the message to stderr, but this could be printed over by our UI
framework so the user would not see it. In general, we disallow the use
of `eprintln!()` in this part of the code for exactly this reason,
though this was suppressed by an `#[allow(clippy::print_stderr)]`.

This attempts to clean things up by changing `handle_event()` and
`handle_tui_event()` to return a `Result<AppRunControl>` instead of a
`Result<bool>`, which is a new type introduced in this PR (and depends
on `ExitReason`, also a new type):

```rust
#[derive(Debug)]
pub(crate) enum AppRunControl {
    Continue,
    Exit(ExitReason),
}

#[derive(Debug, Clone)]
pub enum ExitReason {
    UserRequested,
    Fatal(String),
}
```

This makes it possible to exit the primary control flow of the TUI with
richer information. This PR adds `ExitReason` to the existing
`AppExitInfo` struct and updates `handle_app_exit()` to print the error
and exit code `1` in the event of `ExitReason::Fatal`.

I tried to create an integration test for this, but it was a bit
involved, so I published it as a separate PR:
https://github.com/openai/codex/pull/9166. For this PR, please have
faith in my manual testing!

Fixes https://github.com/openai/codex/issues/8803.




---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/9011).
* #9166
* __->__ #9011

codex-rs/cli/src/main.rs

@@ -26,6 +26,7 @@ use codex_execpolicy::ExecPolicyCheckCommand;
 use codex_responses_api_proxy::Args as ResponsesApiProxyArgs;
 use codex_tui::AppExitInfo;
 use codex_tui::Cli as TuiCli;
+use codex_tui::ExitReason;
 use codex_tui::update_action::UpdateAction;
 use codex_tui2 as tui2;
 use owo_colors::OwoColorize;
@@ -353,6 +354,14 @@ fn format_exit_messages(exit_info: AppExitInfo, color_enabled: bool) -> Vec<Stri
 
 /// Handle the app exit and print the results. Optionally run the update action.
 fn handle_app_exit(exit_info: AppExitInfo) -> anyhow::Result<()> {
+    match exit_info.exit_reason {
+        ExitReason::Fatal(message) => {
+            eprintln!("ERROR: {message}");
+            std::process::exit(1);
+        }
+        ExitReason::UserRequested => { /* normal exit */ }
+    }
+
     let update_action = exit_info.update_action;
     let color_enabled = supports_color::on(Stream::Stdout).is_some();
     for line in format_exit_messages(exit_info, color_enabled) {
@@ -947,6 +956,7 @@ mod tests {
             token_usage,
             thread_id: conversation.map(ThreadId::from_string).map(Result::unwrap),
             update_action: None,
+            exit_reason: ExitReason::UserRequested,
         }
     }
 
@@ -956,6 +966,7 @@ mod tests {
             token_usage: TokenUsage::default(),
             thread_id: None,
             update_action: None,
+            exit_reason: ExitReason::UserRequested,
         };
         let lines = format_exit_messages(exit_info, false);
         assert!(lines.is_empty());