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-protocol/src/protocol/v2.rs

@@ -2107,6 +2107,16 @@ pub struct DeprecationNoticeNotification {
     pub details: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ConfigWarningNotification {
+    /// Concise summary of the warning.
+    pub summary: String,
+    /// Optional extra guidance or error details.
+    pub details: Option<String>,
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;