[tui] Support /mcp command (#2430)

## Summary
Adds a `/mcp` command to list active tools. We can extend this command
to allow configuration of MCP tools, but for now a simple list command
will help debug if your config.toml and your tools are working as
expected.

codex-rs/tui/src/chatwidget.rs

@@ -18,6 +18,7 @@ use codex_core::protocol::ExecApprovalRequestEvent;
 use codex_core::protocol::ExecCommandBeginEvent;
 use codex_core::protocol::ExecCommandEndEvent;
 use codex_core::protocol::InputItem;
+use codex_core::protocol::McpListToolsResponseEvent;
 use codex_core::protocol::McpToolCallBeginEvent;
 use codex_core::protocol::McpToolCallEndEvent;
 use codex_core::protocol::Op;
@@ -647,6 +648,7 @@ impl ChatWidget<'_> {
             EventMsg::McpToolCallBegin(ev) => self.on_mcp_tool_call_begin(ev),
             EventMsg::McpToolCallEnd(ev) => self.on_mcp_tool_call_end(ev),
             EventMsg::GetHistoryEntryResponse(ev) => self.on_get_history_entry_response(ev),
+            EventMsg::McpListToolsResponse(ev) => self.on_list_mcp_tools(ev),
             EventMsg::ShutdownComplete => self.on_shutdown_complete(),
             EventMsg::TurnDiff(TurnDiffEvent { unified_diff }) => self.on_turn_diff(unified_diff),
             EventMsg::BackgroundEvent(BackgroundEventEvent { message }) => {
@@ -685,6 +687,14 @@ impl ChatWidget<'_> {
         ));
     }
 
+    pub(crate) fn add_mcp_output(&mut self) {
+        if self.config.mcp_servers.is_empty() {
+            self.add_to_history(&history_cell::empty_mcp_output());
+        } else {
+            self.submit_op(Op::ListMcpTools);
+        }
+    }
+
     /// Forward file-search results to the bottom pane.
     pub(crate) fn apply_file_search_result(&mut self, query: String, matches: Vec<FileMatch>) {
         self.bottom_pane.on_file_search_result(query, matches);
@@ -726,6 +736,10 @@ impl ChatWidget<'_> {
         }
     }
 
+    fn on_list_mcp_tools(&mut self, ev: McpListToolsResponseEvent) {
+        self.add_to_history(&history_cell::new_mcp_tools_output(&self.config, ev.tools));
+    }
+
     /// Programmatically submit a user text message as if typed in the
     /// composer. The text will be added to conversation history and sent to
     /// the agent.