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
4bede272cf