fix: propagate model through credential pool path + add tests

The cherry-picked fix from PR #7916 placed model propagation after
the credential pool early-return in _resolve_named_custom_runtime(),
making it dead code when a pool is active (which happens whenever
custom_providers has an api_key that auto-seeds the pool).

- Inject model into pool_result before returning
- Add 5 regression tests covering direct path, pool path, empty
  model, and absent model scenarios
- Add 'model' to _VALID_CUSTOM_PROVIDER_FIELDS for config validation

hermes_cli/runtime_provider.py

@@ -332,6 +332,11 @@ def _resolve_named_custom_runtime(
     # Check if a credential pool exists for this custom endpoint
     pool_result = _try_resolve_from_custom_pool(base_url, "custom", custom_provider.get("api_mode"))
     if pool_result:
+        # Propagate the model name even when using pooled credentials —
+        # the pool doesn't know about the custom_providers model field.
+        model_name = custom_provider.get("model")
+        if model_name:
+            pool_result["model"] = model_name
         return pool_result
 
     api_key_candidates = [