docs(a2a): align dependency versions

[FanouZeng-TT]

Description

The dependency summary in a2a/SKILLS.md no longer matches either package manifest:

# a2a/SKILLS.md
- google-adk[a2a]>=1.22.0
- vite ^6.2.0

# actual manifests
- google-adk[a2a]>=1.27.4
- vite ^8.0.16

The backend lockfile's root package metadata also records ADK >=1.27.4. Since the sections are explicitly labeled from backend pyproject.toml and frontend package.json, the older values give readers the wrong supported dependency versions.

Fix: align both documented dependency versions with their manifests.

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 the backend pyproject.toml and frontend package.json, then asserted both dependency versions appear in a2a/SKILLS.md
• uvx pre-commit run --all-files
• git diff --check