chore: import upstream snapshot with attribution

tests/unit/headers-normalize.test.ts

@@ -0,0 +1,70 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { normalizeHeaders, getHeader } = await import("../../open-sse/utils/headers.ts");
+
+test("normalizeHeaders flattens a global Headers instance to plain object with lower-cased keys", () => {
+  const h = new Headers({
+    "Content-Type": "application/json",
+    "Retry-After": "10",
+    "X-Custom": "value",
+  });
+
+  const plain = normalizeHeaders(h);
+
+  assert.deepEqual(plain, {
+    "content-type": "application/json",
+    "retry-after": "10",
+    "x-custom": "value",
+  });
+});
+
+test("normalizeHeaders accepts a plain object and lower-cases its keys", () => {
+  const plain = normalizeHeaders({
+    "X-Codex-5h-Usage": "42",
+    "Retry-After": "5",
+  });
+
+  assert.equal(plain["x-codex-5h-usage"], "42");
+  assert.equal(plain["retry-after"], "5");
+});
+
+test("normalizeHeaders returns {} for null / undefined / empty inputs", () => {
+  assert.deepEqual(normalizeHeaders(null), {});
+  assert.deepEqual(normalizeHeaders(undefined), {});
+  assert.deepEqual(normalizeHeaders({}), {});
+});
+
+test("normalizeHeaders survives an object that throws on forEach (cross-undici-instance simulation)", () => {
+  // Simulate the failure mode described in #2751: an object that *looks* like Headers
+  // but throws when .forEach is called (because the private #headers slot belongs to a
+  // different undici copy). The helper must fall back to entries(), then plain enum.
+  const throwingForEach = {
+    forEach: () => {
+      throw new TypeError("Cannot read private member #headers");
+    },
+    entries: () => [
+      ["X-Survived", "yes"],
+      ["Retry-After", "1"],
+    ][Symbol.iterator](),
+  };
+
+  const plain = normalizeHeaders(throwingForEach as unknown as Headers);
+  assert.equal(plain["x-survived"], "yes");
+  assert.equal(plain["retry-after"], "1");
+});
+
+test("normalizeHeaders falls back to Object.entries when neither forEach nor entries works", () => {
+  const noIterators = { "X-Plain": "ok", "Other": "value" };
+  const plain = normalizeHeaders(noIterators);
+  assert.equal(plain["x-plain"], "ok");
+  assert.equal(plain["other"], "value");
+});
+
+test("getHeader returns the value (case-insensitive) or null", () => {
+  const h = new Headers({ "Retry-After": "30" });
+  assert.equal(getHeader(h, "retry-after"), "30");
+  assert.equal(getHeader(h, "Retry-After"), "30");
+  assert.equal(getHeader(h, "missing"), null);
+  assert.equal(getHeader(null, "any"), null);
+});