test(sse): drop the dead collectSSE helper that reds the lint gate

#12151 landed with a never-called collectSSE helper in
stream-passthrough-usage-estimation.test.ts; its three unused
identifiers (collectSSE / stream / writable) fail
'npm run lint:json -- --max-warnings 0', so the 'No new ESLint
warnings' job reds every PR merge-ref cut from the current tip
(first seen on PR #11564's post-sync run). createSSEStream and
parseSSEUsage are actually used and stay; only the dead closure
goes. File suite still passes 6/6.

tests/unit/stream-passthrough-usage-estimation.test.ts

@@ -34,24 +34,6 @@ test("passthrough no fake: tool_only contentLength==0 -> no estimate (tool_calls
 
 import { createSSEStream } from "../../open-sse/utils/stream.ts";
 
-function collectSSE(stream: TransformStream<Uint8Array, Uint8Array>) {
-  return async (writable: WritableStream<Uint8Array>, readable: ReadableStream<Uint8Array>) => {
-    const chunks: string[] = [];
-    const decoder = new TextDecoder();
-    const reader = readable.getReader();
-    try {
-      while (true) {
-        const { done, value } = await reader.read();
-        if (done) break;
-        chunks.push(decoder.decode(value, { stream: true }));
-      }
-    } finally {
-      reader.releaseLock();
-    }
-    return chunks.join("");
-  };
-}
-
 function parseSSEUsage(sseText: string): unknown[] {
   return sseText
     .split("\n\n")