Handle malformed agent role definitions nonfatally (#14488)

## Summary
- make malformed agent role definitions nonfatal during config loading
- drop invalid agent roles and record warnings in `startup_warnings`
- forward startup warnings through app-server `configWarning`
notifications

## Testing
- `cargo test -p codex-core agent_role_ -- --nocapture`
- `just fix -p codex-core`
- `just fmt`
- `cargo test -p codex-app-server config_warning -- --nocapture`

Co-authored-by: Codex <noreply@openai.com>

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

@@ -468,6 +468,14 @@ pub async fn run_main_with_transport(
     if let Some(warning) = project_config_warning(&config) {
         config_warnings.push(warning);
     }
+    for warning in &config.startup_warnings {
+        config_warnings.push(ConfigWarningNotification {
+            summary: warning.clone(),
+            details: None,
+            path: None,
+            range: None,
+        });
+    }
 
     let feedback = CodexFeedback::new();