[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/mcp-server/src/codex_tool_runner.rs

@@ -263,6 +263,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::AgentReasoningSectionBreak(_)
                     | EventMsg::McpToolCallBegin(_)
                     | EventMsg::McpToolCallEnd(_)
+                    | EventMsg::McpListToolsResponse(_)
                     | EventMsg::ExecCommandBegin(_)
                     | EventMsg::ExecCommandOutputDelta(_)
                     | EventMsg::ExecCommandEnd(_)