chore: strip per-request DEBUG logs from chat path

Remove [DBG:STREAM] chunk/pipe/EOF instrumentation and [DBG:SSE] flush +
fetch success lines that spam the dev console every request. Keep the stream
stall watchdog and the fetch-error (✖) diagnostics.

open-sse/utils/stream.js

@@ -338,8 +338,6 @@ export function createSSEStream(options = {}) {
     },
 
     flush(controller) {
-      const evtSummary = Object.entries(eventTypeCounts).map(([k, v]) => `${k}=${v}`).join(",") || "none";
-      dbg("SSE", `flush | provider=${provider} | model=${model} | recvLines=${sseLineCount} | emitted=${sseEmittedCount} | events=[${evtSummary}]`);
       trackPendingRequest(model, provider, connectionId, false);
       try {
         const remaining = decoder.decode();