fix(codex): propagate envelope usage through legacy chat controller conversion paths

- Capture response.usage before content schema conversion in chat controller
- Thread envelope usage into _ensure_openai_chat_schema for tool-call,
  parsed-content, and plain-text fallback branches
- Remove hardcoded zero-usage fallback dicts that masked real usage data
- Add regression test verifying usage survives ChatController passthrough
- Extend demo script to prove ChatController legacy path preserves usage
- Fix type annotations in usage summary test and demo processor interface

Author: Mateusz