Add first-class ChatGPT subscription provider support

demos/llm_routing/chatgpt_subscription/chat.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+"""Interactive chat with a model through Plano using the OpenAI SDK."""
+
+import sys
+from openai import OpenAI
+
+client = OpenAI(base_url="http://localhost:12000/v1", api_key="unused")
+
+
+def run_chat(model):
+    print(f"Chatting with {model} via Plano (Ctrl+C to quit)\n")
+    history = []
+    while True:
+        try:
+            user_input = input("you> ")
+        except (KeyboardInterrupt, EOFError):
+            print("\nbye")
+            break
+        if not user_input.strip():
+            continue
+
+        history.append({"role": "user", "content": user_input})
+
+        stream = client.responses.create(model=model, input=history, stream=True)
+        print(f"{model}> ", end="", flush=True)
+        full = ""
+        for event in stream:
+            if event.type == "response.output_text.delta":
+                print(event.delta, end="", flush=True)
+                full += event.delta
+        print()
+
+        history.append({"role": "assistant", "content": full})
+
+
+if __name__ == "__main__":
+    model = sys.argv[1] if len(sys.argv) > 1 else "gpt-5.2"
+    run_chat(model)