* fix(api): forward real LLM usage in /v1/chat/completions response
_chat_completion_response() hardcoded prompt_tokens/completion_tokens
to zero. Now reads agent_loop._last_usage (set by process_direct
after every LLM call) and forwards the actual prompt/completion counts.
Streaming path is unchanged; usage is only surfaced in non-streaming
responses for now.
Fixes#4309
* fix: use defensive getattr for _last_usage and add it to all test mock agents
- Use getattr(agent_loop, '_last_usage', None) in server.py for safety
- Add _last_usage = {} to mock agents in test_api_attachment.py and test_api_stream.py
- Prevents AttributeError/500 when mock agents don't have the attribute
* fix(api): preserve provider total usage
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Co-authored-by: michaelxer <michaelxer@users.noreply.github.com>
Co-authored-by: Xubin Ren <52506698+Re-bin@users.noreply.github.com>
