When a guardrail blocks a post-call response, the synthetic violation response
reported hard-coded zero usage, discarding the token usage the upstream call
had already consumed.
Fix the root cause rather than re-counting tokens:
- Add an optional `original_response` field to ModifyResponseException.
- The unified guardrail's post-call success hook attaches the blocked LLM
response to the exception.
- The /v1/messages and OpenAI-format (/v1/chat/completions, /v1/completions)
block handlers report `original_response.usage` directly. Pre-call blocks
never invoked the LLM, so usage is zero.
Mock-based tests cover the helper (returns original usage / zero), the success
hook attaching original_response, and the endpoint reporting it end-to-end.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
