fix(model): preserve custom endpoint credentials and accept cloud models not in /v1/models

When switching models on a custom endpoint (ollama-launch):
- Same-provider switches no longer re-resolve credentials (fixes base_url
  being lost for 'custom' provider on subsequent switches)
- Named providers (ollama-launch) are resolved via user_providers so
  switch_model can find their base_url from config
- Models not in the /v1/models probe but present in the user's saved
  provider config are accepted with a warning instead of rejected
- CLI /model and TUI /model both pass user_providers/custom_providers
  to switch_model so the config model list is available for validation

Closes #15088

hermes_cli/models.py

@@ -2622,8 +2622,8 @@ def validate_requested_model(
                 )
 
             return {
-                "accepted": False,
-                "persist": False,
+                "accepted": True,
+                "persist": True,
                 "recognized": False,
                 "message": message,
             }