fix(doctor): skip /models health check for providers that don't support it

Xiaomi MiMo's /v1/models endpoint returns 401 even with a valid API key,
causing hermes doctor to falsely report 'invalid API key'.

Add a `supports_health_check` field to ProviderProfile (default True).
Providers whose /models endpoint doesn't support auth verification can
set it to False. The doctor's dynamic provider discovery now reads this
field instead of hardcoding True.

The xiaomi provider plugin sets supports_health_check=False.

providers/base.py

@@ -40,6 +40,7 @@ class ProviderProfile:
     base_url: str = ""
     models_url: str = ""  # explicit models endpoint; falls back to {base_url}/models
     auth_type: str = "api_key"   # api_key|oauth_device_code|oauth_external|copilot|aws_sdk
+    supports_health_check: bool = True  # False → doctor skips /models probe for this provider
 
     # ── Model catalog ─────────────────────────────────────────
     # fallback_models: curated list shown in /model picker when live fetch fails.