feat: hot reload mcp servers (#8957)

### Summary
* Added `mcpServer/refresh` command to inform app servers and active
threads to refresh mcpServer on next turn event.
* Added `pending_mcp_server_refresh_config` to codex core so that if the
value is populated, we reinitialize the mcp server manager on the thread
level.
* The config is updated on `mcpServer/refresh` command which we iterate
through threads and provide with the latest config value after last
write.

codex-rs/core/src/thread_manager.rs

@@ -21,6 +21,7 @@ use crate::skills::SkillsManager;
 use codex_protocol::ThreadId;
 use codex_protocol::openai_models::ModelPreset;
 use codex_protocol::protocol::InitialHistory;
+use codex_protocol::protocol::McpServerRefreshConfig;
 use codex_protocol::protocol::Op;
 use codex_protocol::protocol::RolloutItem;
 use codex_protocol::protocol::SessionSource;
@@ -30,6 +31,7 @@ use std::sync::Arc;
 #[cfg(any(test, feature = "test-support"))]
 use tempfile::TempDir;
 use tokio::sync::RwLock;
+use tracing::warn;
 
 /// Represents a newly created Codex thread (formerly called a conversation), including the first event
 /// (which is [`EventMsg::SessionConfigured`]).
@@ -144,6 +146,27 @@ impl ThreadManager {
         self.state.threads.read().await.keys().copied().collect()
     }
 
+    pub async fn refresh_mcp_servers(&self, refresh_config: McpServerRefreshConfig) {
+        let threads = self
+            .state
+            .threads
+            .read()
+            .await
+            .values()
+            .cloned()
+            .collect::<Vec<_>>();
+        for thread in threads {
+            if let Err(err) = thread
+                .submit(Op::RefreshMcpServers {
+                    config: refresh_config.clone(),
+                })
+                .await
+            {
+                warn!("failed to request MCP server refresh: {err}");
+            }
+        }
+    }
+
     pub async fn get_thread(&self, thread_id: ThreadId) -> CodexResult<Arc<CodexThread>> {
         self.state.get_thread(thread_id).await
     }