docs(a2a): update Vite startup output

[FanouZeng-TT]

Description

The A2A quick start shows VITE v5.x.x ready as the expected chat-client
startup output, while a2a/chat-client/package.json and its lockfile use Vite
^8.0.16. Running npm run dev reports VITE v8.0.16, so the documented
major version no longer matches the sample's controlled dependency.

Fix: update the expected startup line to VITE v8.x.x ready, retaining
wildcards for minor and patch releases.

Category (Required)

• Core Protocol: Protocol specification changes. (Requires Technical Council approval)
• Governance/Contributing: Governance or contribution process changes. (Requires Governance Council approval)
• Capability: Capability specification changes. (Requires Maintainer approval)
• Documentation: Documentation updates. (Requires Maintainer approval)
• Infrastructure: Build, CI, tooling, or release changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency, compatibility, or routine maintenance 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): Repository-wide health files and configuration. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide.
• 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 run dev: VITE v8.0.16, local URL http://localhost:3000/
uvx pre-commit run --all-files: passed
git diff --check: passed