chore: import upstream snapshot with attribution

tests/unit/chatcore-stream-finalize.test.ts

@@ -0,0 +1,47 @@
+// tests/unit/chatcore-stream-finalize.test.ts
+// Characterization of wrapReadableStreamWithFinalize — the stream finalize wrapper extracted from
+// handleChatCore (chatCore god-file decomposition, #3501). Locks: finalize runs exactly once on
+// full drain, on cancel, and is not double-invoked.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { wrapReadableStreamWithFinalize } from "../../open-sse/handlers/chatCore/streamFinalize.ts";
+
+function streamOf(chunks: unknown[]): ReadableStream {
+  let i = 0;
+  return new ReadableStream({
+    pull(controller) {
+      if (i < chunks.length) {
+        controller.enqueue(chunks[i++]);
+      } else {
+        controller.close();
+      }
+    },
+  });
+}
+
+test("finalize runs exactly once after the stream is fully drained", async () => {
+  let calls = 0;
+  const wrapped = wrapReadableStreamWithFinalize(streamOf(["a", "b"]), () => {
+    calls++;
+  });
+  const reader = wrapped.getReader();
+  const seen: unknown[] = [];
+  while (true) {
+    const { done, value } = await reader.read();
+    if (done) break;
+    seen.push(value);
+  }
+  assert.deepEqual(seen, ["a", "b"]);
+  assert.equal(calls, 1);
+});
+
+test("finalize runs exactly once on cancel", async () => {
+  let calls = 0;
+  const wrapped = wrapReadableStreamWithFinalize(streamOf(["a", "b", "c"]), () => {
+    calls++;
+  });
+  const reader = wrapped.getReader();
+  await reader.read();
+  await reader.cancel("done early");
+  assert.equal(calls, 1);
+});