chore: rename AI Gateway → Vercel AI Gateway, move Xiaomi to #5 (#9326)

- Rename 'AI Gateway' to 'Vercel AI Gateway' across auth, models,
  doctor, setup, and tests.
- Move Xiaomi MiMo to position #5 in the provider picker.

tests/hermes_cli/test_api_key_providers.py

@@ -44,7 +44,7 @@ class TestProviderRegistry:
         ("kimi-coding", "Kimi / Moonshot", "api_key"),
         ("minimax", "MiniMax", "api_key"),
         ("minimax-cn", "MiniMax (China)", "api_key"),
-        ("ai-gateway", "AI Gateway", "api_key"),
+        ("ai-gateway", "Vercel AI Gateway", "api_key"),
         ("kilocode", "Kilo Code", "api_key"),
     ])
     def test_provider_registered(self, provider_id, name, auth_type):