Silence benign teardown errors from TCPConn.WriteTo (Go 1.27)

[ljzczh]

Newer Go added io.WriterTo support to *net.TCPConn, so io.Copy now prefers WriteTo and wraps teardown errors as OpError{Op: "writeto"} instead of read. These are normal connection-close races (e.g. WebSocket client disconnects), but they fall through the existing readfrom/read filter and get logged as ERROR. This adds writeto to the ignored ops, consistent with the existing intent.