fix: extract custom_provider_slug() helper, harden gateway test

- Add custom_provider_slug() to hermes_cli/providers.py as the single
  source of truth for building 'custom:<name>' slugs.
- Use it in resolve_custom_provider() and list_authenticated_providers()
  instead of duplicated inline slug construction.
- Add _session_model_overrides and _voice_mode to gateway test runner
  for object.__new__() safety.

hermes_cli/model_switch.py

@@ -25,6 +25,7 @@ from dataclasses import dataclass
 from typing import List, NamedTuple, Optional
 
 from hermes_cli.providers import (
+    custom_provider_slug,
     determine_api_mode,
     get_label,
     is_aggregator,
@@ -887,7 +888,7 @@ def list_authenticated_providers(
             if not display_name or not api_url:
                 continue
 
-            slug = "custom:" + display_name.lower().replace(" ", "-")
+            slug = custom_provider_slug(display_name)
             if slug in seen_slugs:
                 continue