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/common/mcp_process.rs

@@ -295,6 +295,11 @@ impl McpProcess {
         self.send_request("getUserAgent", None).await
     }
 
+    /// Send a `userInfo` JSON-RPC request.
+    pub async fn send_user_info_request(&mut self) -> anyhow::Result<i64> {
+        self.send_request("userInfo", None).await
+    }
+
     /// Send a `listConversations` JSON-RPC request.
     pub async fn send_list_conversations_request(
         &mut self,