chore: import upstream snapshot with attribution

apps/sim/lib/copilot/tools/client/trace-context.ts

@@ -0,0 +1,17 @@
+// Browser-side W3C traceparent holder for the active copilot chat.
+// Module-level singleton because client tool callbacks fire from deep
+// inside runtime code that can't thread a React ref. The browser only
+// has one active chat at a time (gated by the stop-barrier), so a
+// singleton is safe.
+
+let currentTraceparent: string | undefined
+
+export function setCurrentChatTraceparent(value: string | undefined): void {
+  currentTraceparent = value
+}
+
+// `fetch` header spread: `headers: { ...traceparentHeader(), ... }`.
+export function traceparentHeader(): Record<string, string> {
+  const tp = currentTraceparent
+  return tp ? { traceparent: tp } : {}
+}