Model-level guardrails (litellm_params.guardrails on a deployment) were
only merged into request metadata in the streaming post_call path
(async_post_call_streaming_hook) but not in the non-streaming path
(post_call_success_hook). This caused should_run_guardrail to skip the
guardrail because the guardrail name was never added to metadata.guardrails.
Add the same _check_and_merge_model_level_guardrails call to
post_call_success_hook before the guardrail callback loop, mirroring the
streaming path.
Fixes model-level guardrails silently not firing for non-streaming
post_call requests.
