Render GTM fallback without buffering HTML responses

[Copilot]

GTM noscript injection buffered and rewrote every HTML response, disabling streaming and increasing latency and memory usage.

• Middleware
  • Restore pass-through response handling.
• Rendering
  • Add a server-rendered GtmNoscript Astro component.
  • Include it in standalone and injected documentation layouts.
  • Preserve environment-specific GTM container resolution.