feat(chatgpt): list ChatGPT (OAuth) in the Add Model provider dropdown

The Add Model form reads its provider list from
``/public/providers/fields`` (served from
``public_endpoints/provider_create_fields.json``), not from the
frontend ``Providers`` enum directly. Add a ChatGPT entry there with a
single required ``api_key`` field pre-filled with ``oauth:`` so the
admin's workflow is:

1. Credentials → Add Credential → ChatGPT (OAuth) → sign in → credential
   named ``my-chatgpt``
2. Add Model → provider: ChatGPT (OAuth) → model name → api_key field
   shows ``oauth:`` placeholder + tooltip explaining the convention →
   admin types ``oauth:my-chatgpt`` → save

At request time the ChatGPT responses transformation recognises the
``oauth:`` prefix and routes through ``DBAuthenticator`` for the stored
credential. Copilot was already in the JSON so its Add Model flow
already works.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

litellm/proxy/public_endpoints/provider_create_fields.json

@@ -570,6 +570,24 @@
     ],
     "default_model_placeholder": "gpt-3.5-turbo"
   },
+  {
+    "provider": "ChatGPT",
+    "provider_display_name": "ChatGPT (OAuth)",
+    "litellm_provider": "chatgpt",
+    "credential_fields": [
+      {
+        "key": "api_key",
+        "label": "OAuth credential reference",
+        "placeholder": "oauth:my-chatgpt",
+        "tooltip": "Reference a stored ChatGPT OAuth credential by name, prefixed with 'oauth:'. Create the credential first via Credentials \u2192 Add Credential \u2192 ChatGPT (OAuth).",
+        "required": true,
+        "field_type": "text",
+        "options": null,
+        "default_value": "oauth:"
+      }
+    ],
+    "default_model_placeholder": "gpt-5.3-codex"
+  },
   {
     "provider": "CLARIFAI",
     "provider_display_name": "Clarifai",