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/executors/base.js

@@ -139,8 +139,6 @@ export class BaseExecutor {
 
       try {
         const bodyStr = JSON.stringify(transformedBody);
-        const fetchT0 = Date.now();
-        dbg("FETCH", `${this.provider.toUpperCase()} → ${url} | body=${bodyStr.length}B | connectTimeout=${timeoutMs}ms`);
         const response = await proxyAwareFetch(url, {
           method: "POST",
           headers,
@@ -148,9 +146,6 @@ export class BaseExecutor {
           signal: mergedSignal
         }, proxyOptions);
         clearTimeout(connectTimer);
-        const ct = response.headers?.get?.("content-type") || "";
-        const cl = response.headers?.get?.("content-length") || "?";
-        dbg("FETCH", `${this.provider.toUpperCase()} ← ${response.status} | ttft=${Date.now() - fetchT0}ms | ct=${ct} | cl=${cl}`);
 
         if (await tryRetry(urlIndex, response.status, `status ${response.status}`, response)) { urlIndex--; continue; }