fix: include custom_providers in /model command listings and resolution

Custom providers defined in config.yaml under  were
completely invisible to the /model command in both gateway (Telegram,
Discord, etc.) and CLI. The provider listing skipped them and explicit
switching via --provider failed with "Unknown provider".

Root cause: gateway/run.py, cli.py, and model_switch.py only read the
 dict from config, ignoring  entirely.

Changes:
- providers.py: add resolve_custom_provider() and extend
  resolve_provider_full() to check custom_providers after user_providers
- model_switch.py: propagate custom_providers through switch_model(),
  list_authenticated_providers(), and get_authenticated_provider_slugs();
  add custom provider section to provider listings
- gateway/run.py: read custom_providers from config, pass to all
  model-switch calls
- cli.py: hoist config loading, pass custom_providers to listing and
  switch calls

Tests: 4 new regression tests covering listing, resolution, and gateway
command handler. All 71 tests pass.

gateway/run.py

@@ -3546,6 +3546,7 @@ class GatewayRunner:
         current_base_url = ""
         current_api_key = ""
         user_provs = None
+        custom_provs = None
         config_path = _hermes_home / "config.yaml"
         try:
             if config_path.exists():
@@ -3557,6 +3558,7 @@ class GatewayRunner:
                     current_provider = model_cfg.get("provider", current_provider)
                     current_base_url = model_cfg.get("base_url", "")
                 user_provs = cfg.get("providers")
+                custom_provs = cfg.get("custom_providers")
         except Exception:
             pass
 
@@ -3584,6 +3586,7 @@ class GatewayRunner:
                     providers = list_authenticated_providers(
                         current_provider=current_provider,
                         user_providers=user_provs,
+                        custom_providers=custom_provs,
                         max_models=50,
                     )
                 except Exception:
@@ -3611,6 +3614,8 @@ class GatewayRunner:
                             current_api_key=_cur_api_key,
                             is_global=False,
                             explicit_provider=provider_slug,
+                            user_providers=user_provs,
+                            custom_providers=custom_provs,
                         )
                         if not result.success:
                             return f"Error: {result.error_message}"
@@ -3689,6 +3694,7 @@ class GatewayRunner:
                 providers = list_authenticated_providers(
                     current_provider=current_provider,
                     user_providers=user_provs,
+                    custom_providers=custom_provs,
                     max_models=5,
                 )
                 for p in providers:
@@ -3718,6 +3724,8 @@ class GatewayRunner:
             current_api_key=current_api_key,
             is_global=persist_global,
             explicit_provider=explicit_provider,
+            user_providers=user_provs,
+            custom_providers=custom_provs,
         )
 
         if not result.success: