fix: fall back to provider's default model when model config is empty (#8303)

When a user configures a provider (e.g. `hermes auth add openai-codex`)
but never selects a model via `hermes model`, the gateway and CLI would
pass an empty model string to the API, causing:
  'Codex Responses request model must be a non-empty string'

Now both gateway (_resolve_session_agent_runtime) and CLI
(_ensure_runtime_credentials) detect an empty model and fill it from
the provider's first catalog entry in _PROVIDER_MODELS. This covers
all providers that have a static model list (openai-codex, anthropic,
gemini, copilot, etc.).

The fix is conservative: it only triggers when model is truly empty
and a known provider was resolved. Explicit model choices are never
overridden.

cli.py

@@ -2735,6 +2735,22 @@ class HermesCLI:
         if runtime_model and isinstance(runtime_model, str):
             self.model = runtime_model
 
+        # If model is still empty (e.g. user ran `hermes auth add openai-codex`
+        # without `hermes model`), fall back to the provider's first catalog
+        # model so the API call doesn't fail with "model must be non-empty".
+        if not self.model and resolved_provider:
+            try:
+                from hermes_cli.models import get_default_model_for_provider
+                _default = get_default_model_for_provider(resolved_provider)
+                if _default:
+                    self.model = _default
+                    logger.info(
+                        "No model configured — defaulting to %s for provider %s",
+                        _default, resolved_provider,
+                    )
+            except Exception:
+                pass
+
         # Normalize model for the resolved provider (e.g. swap non-Codex
         # models when provider is openai-codex).  Fixes #651.
         model_changed = self._normalize_model_for_provider(resolved_provider)