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.

plugins/model-providers/xiaomi/__init__.py

@@ -8,6 +8,7 @@ xiaomi = ProviderProfile(
     aliases=("mimo", "xiaomi-mimo"),
     env_vars=("XIAOMI_API_KEY",),
     base_url="https://api.xiaomimimo.com/v1",
+    supports_health_check=False,  # /v1/models returns 401 even with valid key
 )
 
 register_provider(xiaomi)