Load app-server config through ConfigManager (#18870)

## Summary
- Load app-server startup config through `ConfigManager` instead of
direct `ConfigBuilder` calls.
- Move `ConfigManager` constructor-owned state (`cli_overrides`, runtime
feature map, cloud requirements loader) behind internal manager fields.
- Pass `ConfigManager` into `MessageProcessor` directly instead of
reconstructing it from raw args.

## Tests
- `cargo check -p codex-app-server`
- `cargo test -p codex-app-server`
- `just fix -p codex-app-server`
- `just fmt`

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

@@ -10352,7 +10352,6 @@ mod tests {
     use std::collections::BTreeMap;
     use std::path::PathBuf;
     use std::sync::Arc;
-    use std::sync::RwLock;
     use tempfile::TempDir;
 
     #[test]
@@ -10628,10 +10627,9 @@ mod tests {
         };
         let config_manager = ConfigManager::new(
             temp_dir.path().to_path_buf(),
-            Arc::new(RwLock::new(Vec::new())),
-            Arc::new(RwLock::new(BTreeMap::new())),
+            Vec::new(),
             LoaderOverrides::default(),
-            Arc::new(RwLock::new(CloudRequirementsLoader::default())),
+            CloudRequirementsLoader::default(),
             Arg0DispatchPaths::default(),
             Arc::new(StaticThreadConfigLoader::new(vec![
                 ThreadConfigSource::Session(SessionThreadConfig {