feat: show MCP tool calls in TUI (#836)

Adds logic for the `McpToolCallBegin` and `McpToolCallEnd` events in
`codex-rs/tui/src/chatwidget.rs` so they get entries in the conversation
history in the TUI.

Building on top of https://github.com/openai/codex/pull/829, here is the
result of running:

```
cargo run --bin codex -- 'what is the weather in san francisco tomorrow'
```


![image](https://github.com/user-attachments/assets/db4a79bb-4988-46cb-acb2-446d5ba9e058)

codex-rs/tui/src/chatwidget.rs

@@ -328,6 +328,25 @@ impl ChatWidget<'_> {
                     .record_completed_exec_command(call_id, stdout, stderr, exit_code);
                 self.request_redraw()?;
             }
+            EventMsg::McpToolCallBegin {
+                call_id,
+                server,
+                tool,
+                arguments,
+            } => {
+                self.conversation_history
+                    .add_active_mcp_tool_call(call_id, server, tool, arguments);
+                self.request_redraw()?;
+            }
+            EventMsg::McpToolCallEnd {
+                call_id,
+                success,
+                result,
+            } => {
+                self.conversation_history
+                    .record_completed_mcp_tool_call(call_id, success, result);
+                self.request_redraw()?;
+            }
             event => {
                 self.conversation_history
                     .add_background_event(format!("{event:?}"));