fix(ui-tui): stabilize embedded dashboard chat gateway (#44528)

Cherry-picked from #39840 by @flyinhigh and rebased cleanly on main.

- Defer config fetch in createGatewayEventHandler until gateway.ready to
  avoid render-phase RPC that can mutate transcript state and trigger
  React error 301 in embedded dashboard PTYs.
- Use undici WebSocket fallback when globalThis.WebSocket is unavailable
  (Node attach mode and sidecar mirror sockets).
- Add regression tests for both fixes.

Co-authored-by: flyinhigh <flyinhigh@users.noreply.github.com>

ui-tui/package.json

@@ -22,6 +22,7 @@
     "ink-text-input": "^6.0.0",
     "nanostores": "^1.2.0",
     "react": "^19.2.4",
+    "undici": "^6.25.0",
     "unicode-animations": "^1.0.3"
   },
   "devDependencies": {