chore: import upstream snapshot with attribution

open-sse/utils/headers.ts

@@ -0,0 +1,61 @@
+/**
+ * normalizeHeaders — cross-undici-instance Headers normalizer.
+ *
+ * On Node 24, accessing an undici-backed `Headers` object that was constructed
+ * by a *different* undici copy (e.g. a mock global `Response` in tests, or a
+ * version mismatch between undici instances bundled into different workspaces)
+ * throws `Cannot read private member #headers` because undici guards its internal
+ * state with private class fields.
+ *
+ * This helper avoids that crash by iterating through the public iterable API
+ * (`forEach`, then `entries`) first, falling back to plain-object enumeration
+ * for any non-standard map-like value.
+ *
+ * Always returns a plain `Record<string, string>` with lower-cased keys so
+ * callers never need to worry about header case sensitivity.
+ */
+export function normalizeHeaders(h: unknown): Record<string, string> {
+  if (!h) return {};
+  const out: Record<string, string> = {};
+
+  // Preferred path: forEach avoids creating an iterator object
+  try {
+    if (typeof (h as Headers).forEach === "function") {
+      (h as Headers).forEach((value: string, key: string) => {
+        out[key.toLowerCase()] = value;
+      });
+      return out;
+    }
+  } catch {
+    // fall through — different undici instance, try next approach
+  }
+
+  // Second path: entries() iterator
+  try {
+    if (typeof (h as Headers).entries === "function") {
+      for (const [k, v] of (h as Headers).entries()) {
+        out[k.toLowerCase()] = String(v);
+      }
+      return out;
+    }
+  } catch {
+    // fall through — try plain object enumeration
+  }
+
+  // Final fallback: treat as plain object
+  if (typeof h === "object") {
+    for (const [k, v] of Object.entries(h as Record<string, unknown>)) {
+      out[k.toLowerCase()] = String(v ?? "");
+    }
+  }
+  return out;
+}
+
+/**
+ * Convenience wrapper: get a header value from any headers-like object.
+ * Returns null (same contract as `Headers.prototype.get`) when absent.
+ */
+export function getHeader(h: unknown, name: string): string | null {
+  const plain = normalizeHeaders(h);
+  return plain[name.toLowerCase()] ?? null;
+}