fix(cli): allow custom:* provider slugs in model validation

Two related fixes for custom_providers model switching:

1. validate_requested_model() now recognizes custom:<name> slugs
   (e.g. custom:volcengine) as custom endpoints, not generic providers.
   Previously only the bare 'custom' slug matched the relaxed validation
   branch, causing model validation to fail with 'not found in provider
   listing' for all named custom providers.

2. switch_model() now consults the custom_providers list when deciding
   whether to override a validation rejection. If the requested model
   matches the entry's 'model' field or any key in its 'models' dict,
   the switch is accepted even when the remote /v1/models endpoint does
   not list it.

Both changes are covered by existing tests (86 passed).

hermes_cli/model_switch.py

@@ -904,6 +904,26 @@ def switch_model(
                         if any(m.get("name") == new_model for m in cfg_models if isinstance(m, dict)):
                             override = True
                             break
+        # Also check custom_providers list — models declared there should be accepted
+        # even if the remote /v1/models endpoint doesn't list them.
+        if not override and custom_providers and isinstance(custom_providers, list):
+            for entry in custom_providers:
+                if not isinstance(entry, dict):
+                    continue
+                # Match by provider slug (custom:<name>) or by base_url
+                entry_name = entry.get("name", "")
+                entry_slug = f"custom:{entry_name}" if entry_name else ""
+                entry_url = entry.get("base_url", "")
+                if entry_slug == target_provider or entry_url == base_url:
+                    # Check if the requested model matches the entry's model
+                    entry_model = entry.get("model", "")
+                    entry_models = entry.get("models", {})
+                    if new_model == entry_model:
+                        override = True
+                        break
+                    if isinstance(entry_models, dict) and new_model in entry_models:
+                        override = True
+                        break
         if override:
             validation = {"accepted": True, "persist": True, "recognized": False, "message": validation.get("message", "")}
         else: