fix(models): pass model.base_url to fetch_models in /model picker

The /model interactive picker resolved a base_url from user credentials
but never passed it to ProviderProfile.fetch_models(), causing the
picker to always query the provider's hardcoded default endpoint
instead of the user's custom URL (e.g. a company litellm proxy).

- providers/base.py: add optional base_url parameter to fetch_models()
- hermes_cli/models.py: pass resolved base_url to fetch_models()
- Update all subclass overrides for signature compatibility
- Add 6 regression tests covering override, fallback, and integration

plugins/model-providers/custom/__init__.py

@@ -43,12 +43,13 @@ class CustomProfile(ProviderProfile):
         self,
         *,
         api_key: str | None = None,
+        base_url: str | None = None,
         timeout: float = 8.0,
     ) -> list[str] | None:
         """Custom/Ollama: base_url is user-configured; fetch if set."""
-        if not self.base_url:
+        if not (base_url or self.base_url):
             return None
-        return super().fetch_models(api_key=api_key, timeout=timeout)
+        return super().fetch_models(api_key=api_key, base_url=base_url, timeout=timeout)
 
 
 custom = CustomProfile(