feat(showcase/langgraph-python): D6 conveyance shim + copilotkit 0.1.93 bump

- Add _header_forwarding_middleware.py paired with reasoning / subagent /
  tool-rendering-reasoning-chain agent edits so D6 fixture-matching sees
  the inflight x-aimock-context on outbound LLM calls
- Bump copilotkit 0.1.92 → 0.1.93 in requirements.txt; regenerate
  package-lock.json against the post-npm-ci tree (sibling of 03bed3b,
  which switched this integration to npm ci)
- Drop the stale pnpm-lock.yaml left behind by the npm ci migration —
  Dockerfile uses 'npm ci --legacy-peer-deps' and every other showcase
  integration committed only package-lock.json after 03bed3b. Removing
  the orphan prevents npm/pnpm tooling drift from re-resolving against it.

Author: jpr5