add(core): managed config (#3868)

## Summary

- Factor `load_config_as_toml` into `core::config_loader` so config
loading is reusable across callers.
- Layer `~/.codex/config.toml`, optional `~/.codex/managed_config.toml`,
and macOS managed preferences (base64) with recursive table merging and
scoped threads per source.

## Config Flow

```
Managed prefs (macOS profile: com.openai.codex/config_toml_base64)
                               ▲
                               │
~/.codex/managed_config.toml   │  (optional file-based override)
                               ▲
                               │
                ~/.codex/config.toml (user-defined settings)
```

- The loader searches under the resolved `CODEX_HOME` directory
(defaults to `~/.codex`).
- Managed configs let administrators ship fleet-wide overrides via
device profiles which is useful for enforcing certain settings like
sandbox or approval defaults.
- For nested hash tables: overlays merge recursively. Child tables are
merged key-by-key, while scalar or array values replace the prior layer
entirely. This lets admins add or tweak individual fields without
clobbering unrelated user settings.

codex-rs/mcp-server/src/message_processor.rs

@@ -342,7 +342,10 @@ impl MessageProcessor {
     async fn handle_tool_call_codex(&self, id: RequestId, arguments: Option<serde_json::Value>) {
         let (initial_prompt, config): (String, Config) = match arguments {
             Some(json_val) => match serde_json::from_value::<CodexToolCallParam>(json_val) {
-                Ok(tool_cfg) => match tool_cfg.into_config(self.codex_linux_sandbox_exe.clone()) {
+                Ok(tool_cfg) => match tool_cfg
+                    .into_config(self.codex_linux_sandbox_exe.clone())
+                    .await
+                {
                     Ok(cfg) => cfg,
                     Err(e) => {
                         let result = CallToolResult {