[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/exec/src/event_processor_with_human_output.rs

@@ -523,6 +523,9 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             EventMsg::GetHistoryEntryResponse(_) => {
                 // Currently ignored in exec output.
             }
+            EventMsg::McpListToolsResponse(_) => {
+                // Currently ignored in exec output.
+            }
             EventMsg::TurnAborted(abort_reason) => match abort_reason.reason {
                 TurnAbortReason::Interrupted => {
                     ts_println!(self, "task interrupted");