[V16.2+] Router State Header Was Sent But Could Not Be Parsed On Soft Navigation With Dynamic Rendering

The encoding/format of the Next-Router-State-Tree header changed between 16.1 and 16.2. The 16.2 server invokes parseAndValidateFlightRouterState without catching decode/parse errors from older header payloads, so a stale client header triggers an unhandled exception instead of falling back to full navigation.

1. Deploy a Next.js app with a dynamic root layout (e.g. calls cookies()) on 16.1.x. Open it in a browser. 2. Upgrade the same app to 16.2.x and redeploy without refreshing the browser. 3. Click a client-side Link. Server receives a router state header encoded by 16.1, fails to parse, and throws `The router state header was sent but could not be parsed`.

Returning undefined may disable incremental rendering optimizations for affected navigations until the client refreshes. In concurrent or multi-instance deployments, some instances may still run versions that throw, so deploy this change together with a version pin or rollout. Rollback: if this patch causes regressions, revert to the previous throwing version; no data migration is required because the header is stateless.