fix(rest/nodejs): log invalid validation payloads

[FanouZeng-TT]

Description

prettyValidation logged failed JSON validation payloads with:

JSON.stringify(c.req.json(), null, 2)

c.req.json() returns a Promise, and stringifying that Promise produces {}. The warning therefore omitted the invalid request body that it was intended to show.

Fix: serialize the parsed request value already provided as result.data by @hono/zod-validator, and cover the failure path with a regression test that preserves the existing 422 response.

Category (Required)

• Core Protocol: Changes to the core UCP specification. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: Changes to a UCP capability. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: CI/CD or repository infrastructure changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency or repository 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 community health 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.
• 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 (not applicable).

Screenshots / Logs (if applicable)

• npm test: 120 passed
• npm run build: passed
• pre-commit run --all-files: passed