docs(a2a): update Node.js prerequisite

[FanouZeng-TT]

Description

The A2A quick-start guide lists Node.js 18+ as sufficient for the chat client:

- [ ] Node.js 18+

The committed frontend lockfile uses Vite 8.0.16, whose package metadata requires Node.js ^20.19.0 || >=22.12.0. Following the documented prerequisite with Node 18 therefore selects a runtime outside the installed toolchain's supported range.

Fix: update the prerequisite to Node.js 20.x (20.19+) or 22.12+, matching the locked Vite engine.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (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 (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• 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)

• parsed a2a/chat-client/package-lock.json and verified Vite 8.0.16 requires ^20.19.0 || >=22.12.0
• asserted the README prerequisite covers both supported Node ranges
• uvx pre-commit run --all-files
• git diff --check