fix(proxy): strip malformed Next-Router-State-Tree headers

[mattdjenkinson]

Problem

After deployments, clients that had the login page open retain stale router state.
When they navigate, Next.js App Router sends an RSC request with a malformed
Next-Router-State-Tree header — specifically [""] instead of the full routing
tree. Next.js fails to parse this and throws an unhandled error:

Error: The router state header was sent but could not be parsed.

This surfaces on the /(main)/(illustration)/loginname/page route and is tracked
in Sentry:
https://sentry.prod.env.datum.net/organizations/sentry/issues/1918/events/105a342aff4e46f8946d950cb8c6a6bc/

Fix

Added a check in proxy.ts that validates the Next-Router-State-Tree header
before the request reaches the Next.js rendering pipeline. If the value cannot be
parsed as JSON or is structurally incomplete (fewer than 2 elements), the RSC-related
headers (Next-Router-State-Tree, Rsc, Next-Router-Prefetch) are stripped.

Without these headers, Next.js falls back to a full SSR page render, which resets
the client's router state and serves the page correctly.

Impact on normal users

• Normal full page loads — no Next-Router-State-Tree header present, code is a no-op.
• Normal RSC navigations — header parses correctly with 2+ elements, nothing is stripped.
• Stale clients — headers stripped, user gets a full page load instead of an error.