feat: add UserInfo request to JSON-RPC server (#3428)

This adds a simple endpoint that provides the email address encoded in
`$CODEX_HOME/auth.json`.

As noted, for now, we do not hit the server to verify this is the user's
true email address.

codex-rs/mcp-server/tests/suite/mod.rs

@@ -10,3 +10,4 @@ mod list_resume;
 mod login;
 mod send_message;
 mod user_agent;
+mod user_info;