fix: remove unnecessary flush() calls (#2873)

Because we are writing to a pipe, these `flush()` calls are unnecessary,
so removing these saves us one syscall per write in these two cases.

codex-rs/mcp-client/src/mcp_client.rs

@@ -129,10 +129,7 @@ impl McpClient {
                                 error!("failed to write newline to child stdin");
                                 break;
                             }
-                            if stdin.flush().await.is_err() {
-                                error!("failed to flush child stdin");
-                                break;
-                            }
+                            // No explicit flush needed on a pipe; write_all is sufficient.
                         }
                         Err(e) => error!("failed to serialize JSONRPCMessage: {e}"),
                     }