Add first-class ChatGPT subscription provider support

demos/llm_routing/chatgpt_subscription/test_chatgpt.sh

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+#!/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."