stream: optimize single-slot push queue drain

Avoid allocating a new result array when PushQueue drains a single
queued slot. Return that slot directly and keep the existing flattening
path for multiple queued slots.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5
PR-URL: #63274
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ethan Arrowood <ethan@arrowood.dev>

Author: trivikr

lib/internal/streams/iter/push.js

@@ -448,6 +448,10 @@ class PushQueue {
   // ===========================================================================
 
   #drain() {
+    if (this.#slots.length === 1) {
+      return this.#slots.shift();
+    }
+
     const result = [];
     for (let i = 0; i < this.#slots.length; i++) {
       const slot = this.#slots.get(i);