[prompts] Add <environment_context> (#1869)

## Summary
Includes a new user message in the api payload which provides useful
environment context for the model, so it knows about things like the
current working directory and the sandbox.

## Testing
Updated unit tests

codex-rs/mcp-server/tests/send_message.rs

@@ -99,7 +99,7 @@ async fn test_send_message_success() {
         response
     );
     // wait for the server to hear the user message
-    sleep(Duration::from_secs(1));
+    sleep(Duration::from_secs(10));
 
     // Ensure the server and tempdir live until end of test
     drop(server);