Improve Vercel/LangChain integration docs, publish hardening, and deployed /api/chat E2E

[saschabuehrle]

Summary

This PR closes the remaining integration-level gaps for Vercel + LangChain and adds deploy-level validation for real /api/chat paths.

1) Vercel example upgrade (Next.js v2 era alignment)

• Updated examples/vercel-ai-nextjs to current Next.js line (^16.1.6).
• Added webpack-mode scripts (next build --webpack, next dev --webpack) for compatibility with existing custom webpack config.
• Added prebuild/predev workspace build hooks for local monorepo DX.
• Added monorepo alias pinning in next.config.js so example builds resolve local integration/core output instead of stale store artifacts.

2) LangChain/Vercel docs + model references refresh

• Updated integration-facing Anthropic examples to current defaults used by the integration work:
  • drafter: claude-haiku-4-5-20251001
  • verifier: claude-opus-4-6
• Refreshed key docs/examples under:
  • docs/guides/langchain_integration.md
  • packages/langchain-cascadeflow/README.md
  • packages/langchain-cascadeflow/docs/COST_TRACKING.md
  • packages/langchain-cascadeflow/examples/*
  • docs/guides/integrate_fast.md
• Updated LangChain pricebook references for current Anthropic IDs in src/models.ts and src/utils.ts.

3) Publish workflow hardening

• Removed silent publish success behavior by eliminating continue-on-error in npm and PyPI publish steps.
• Added explicit npm version-exists checks before publish, so only already-published versions skip cleanly.
• Added skip-existing: true for PyPI/TestPyPI publish actions.
• File: .github/workflows/publish.yml

4) Real deploy-level E2E workflow

• Added new workflow: .github/workflows/vercel-deploy-e2e.yml
• Flow:
  • create temporary Vercel project
  • disable sandbox deployment protection (ssoProtection) for direct API probing
  • inject OPENAI_API_KEY
  • deploy temporary app
  • call deployed /api/chat
  • validate streamed text response
  • cleanup temporary project
• Added safe secret gating step so workflow skips deploy logic if required secrets are missing.

5) Deployment protection docs

• Added explicit ssoProtection note and real deployed /api/chat smoke-test instructions to:
  • examples/vercel-ai-nextjs/README.md
  • packages/integrations/vercel-ai/README.md
  • docs/guides/integrate_fast.md

Validation

Local:

• pnpm --filter @cascadeflow/langchain build
• pnpm --filter @cascadeflow/langchain test
• pnpm --filter @cascadeflow/langchain typecheck
• pnpm --filter @cascadeflow/vercel-ai test
• pnpm --filter @cascadeflow/core build
• pnpm -C examples/vercel-ai-nextjs build

Real APIs / deployed path:

• Performed real Vercel deploy + real provider call and validated deployed /api/chat stream end-to-end (cascadeflow-ok).

GitHub Actions (new workflow):

• Vercel Deploy E2E run passed on this branch:
  • https://github.com/lemony-ai/cascadeflow/actions/runs/22239335972

Scope / Safety

• Changes are integration/docs/CI focused.
• No core cascade decision logic changes outside integration-facing model reference/pricing metadata alignment in LangChain package.