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/config.py

@@ -1497,7 +1497,7 @@ _KNOWN_ROOT_KEYS = {
 
 # Valid fields inside a custom_providers list entry
 _VALID_CUSTOM_PROVIDER_FIELDS = {
-    "name", "base_url", "api_key", "api_mode", "models",
+    "name", "base_url", "api_key", "api_mode", "model", "models",
     "context_length", "rate_limit_delay",
 }