Improve handling of config and rules errors for app server clients (#9182)

When an invalid config.toml key or value is detected, the CLI currently
just quits. This leaves the VSCE in a dead state.

This PR changes the behavior to not quit and bubble up the config error
to users to make it actionable. It also surfaces errors related to
"rules" parsing.

This allows us to surface these errors to users in the VSCE, like this:

<img width="342" height="129" alt="Screenshot 2026-01-13 at 4 29 22 PM"
src="https://github.com/user-attachments/assets/a79ffbe7-7604-400c-a304-c5165b6eebc4"
/>

<img width="346" height="244" alt="Screenshot 2026-01-13 at 4 45 06 PM"
src="https://github.com/user-attachments/assets/de874f7c-16a2-4a95-8c6d-15f10482e67b"
/>

codex-rs/app-server/src/lib.rs

@@ -1,6 +1,7 @@
 #![deny(clippy::print_stdout, clippy::print_stderr)]
 
 use codex_common::CliConfigOverrides;
+use codex_core::config::Config;
 use codex_core::config::ConfigBuilder;
 use codex_core::config_loader::LoaderOverrides;
 use std::io::ErrorKind;
@@ -10,7 +11,9 @@ use std::path::PathBuf;
 use crate::message_processor::MessageProcessor;
 use crate::outgoing_message::OutgoingMessage;
 use crate::outgoing_message::OutgoingMessageSender;
+use codex_app_server_protocol::ConfigWarningNotification;
 use codex_app_server_protocol::JSONRPCMessage;
+use codex_core::check_execpolicy_for_warnings;
 use codex_feedback::CodexFeedback;
 use tokio::io::AsyncBufReadExt;
 use tokio::io::AsyncWriteExt;
@@ -82,14 +85,38 @@ pub async fn run_main(
         )
     })?;
     let loader_overrides_for_config_api = loader_overrides.clone();
-    let config = ConfigBuilder::default()
+    let mut config_warnings = Vec::new();
+    let config = match ConfigBuilder::default()
         .cli_overrides(cli_kv_overrides.clone())
         .loader_overrides(loader_overrides)
         .build()
         .await
-        .map_err(|e| {
-            std::io::Error::new(ErrorKind::InvalidData, format!("error loading config: {e}"))
-        })?;
+    {
+        Ok(config) => config,
+        Err(err) => {
+            let message = ConfigWarningNotification {
+                summary: "Invalid configuration; using defaults.".to_string(),
+                details: Some(err.to_string()),
+            };
+            config_warnings.push(message);
+            Config::load_default_with_cli_overrides(cli_kv_overrides.clone()).map_err(|e| {
+                std::io::Error::new(
+                    ErrorKind::InvalidData,
+                    format!("error loading default config after config error: {e}"),
+                )
+            })?
+        }
+    };
+
+    if let Ok(Some(err)) =
+        check_execpolicy_for_warnings(&config.features, &config.config_layer_stack).await
+    {
+        let message = ConfigWarningNotification {
+            summary: "Error parsing rules; custom rules not applied.".to_string(),
+            details: Some(err.to_string()),
+        };
+        config_warnings.push(message);
+    }
 
     let feedback = CodexFeedback::new();
 
@@ -127,6 +154,12 @@ pub async fn run_main(
         .with(otel_logger_layer)
         .with(otel_tracing_layer)
         .try_init();
+    for warning in &config_warnings {
+        match &warning.details {
+            Some(details) => error!("{} {}", warning.summary, details),
+            None => error!("{}", warning.summary),
+        }
+    }
 
     // Task: process incoming messages.
     let processor_handle = tokio::spawn({
@@ -140,6 +173,7 @@ pub async fn run_main(
             cli_overrides,
             loader_overrides,
             feedback.clone(),
+            config_warnings,
         );
         async move {
             while let Some(msg) = incoming_rx.recv().await {