fix(rest/nodejs): return UCP envelope for missing orders

[FanouZeng-TT]

Description

OrderService returned a flat body when an order lookup missed:

return c.json({ error: "Order not found" }, 404);

The order REST specification documents not-found responses as the UCP error
envelope (ucp.status: "error" with a typed messages[] entry), and the Node
sample already uses ResourceNotFoundError plus ucpErrorResponse(...) for the
same taxonomy in checkout paths.

Fix: route both missing-order branches through the existing UCP error helper
and cover GET/PUT /orders/:id misses in the envelope regression test.

Category (Required)

• Core Protocol: Changes to the core protocol specification (Requires Technical Council approval)
• Governance/Contributing: Changes to governance, contribution process, or repo management (Requires Governance Council approval)
• Capability: Changes to a capability definition (Requires Maintainer approval)
• Documentation: Documentation-only changes (Requires Maintainer approval)
• Infrastructure: CI/CD, build system, or repository infrastructure changes (Requires DevOps Maintainer approval)
• Maintenance: Dependency updates, refactors, or internal maintenance (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool. (Requires Maintainer approval)
• Community Health (.github): Changes to organization-wide templates/workflows (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and Code of Conduct.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

• npm test — 156/156 passed
• npm run build
• uvx pre-commit run --all-files
• git diff --check