Add first-class ChatGPT subscription provider support (#881)

* Add first-class ChatGPT subscription provider support

* Address PR feedback: move uuid import to top, reuse parsed config in up()

* Add ChatGPT token watchdog for seamless long-lived sessions

* Address PR feedback: error on stream=false for ChatGPT, fix auth file permissions

* Replace ChatGPT watchdog/restart with passthrough_auth

---------

Co-authored-by: Musa Malik <musam@uw.edu>

demos/llm_routing/chatgpt_subscription/test_chatgpt.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+# Test ChatGPT subscription routing through Plano
+# Prerequisites: planoai chatgpt login && planoai up config.yaml
+
+set -e
+
+echo "Testing ChatGPT subscription via Plano Responses API..."
+echo ""
+
+curl -s http://localhost:12000/v1/responses \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "gpt-5.2",
+    "input": "What is 2 + 2? Reply in one word."
+  }' | python3 -m json.tool
+
+echo ""
+echo "Done."