fix(rest/nodejs): align discovery declarations

[FanouZeng-TT]

Description

rest/nodejs/src/api/discovery.ts advertised specification and schema URLs that do not exist in the pinned 2026-04-08 release. For example:

https://ucp.dev/2026-04-08/specification/shopping/checkout -> 404
https://ucp.dev/2026-04-08/schemas/shopping/refund.json  -> 404

The profile also advertised refund, return, and dispute capabilities, but v2026-04-08 has no schemas for them and the Node sample has no corresponding capability implementation.

Fix: use the published service/capability URLs, remove the three unsupported capability declarations, and extend the discovery regression test to assert the advertised capability set and URLs.

Category (Required)

• Core Protocol: Changes to core UCP schemas, specification, or protocol behavior. (Requires Technical Council approval)
• Governance/Contributing: Updates to governance documents, contribution guidelines, or project processes. (Requires Governance Council approval)
• Capability: New or updated capability definitions. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: CI/CD, build tooling, or repository automation. (Requires DevOps Maintainer approval)
• Maintenance: Dependency updates, refactoring, or non-functional changes. (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): Issue templates, code of conduct, or community files. (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 (154/154)
• npm run build
• all 12 advertised ucp.dev/2026-04-08 spec/schema URLs returned HTTP 200
• uvx pre-commit run --all-files
• git diff --check