Add direct zh-CN preview links for PR deployments

[Jiayi-Ye02]

What to build

Every pull request preview deployment should expose a direct Chinese docs preview entry point, so reviewers can open the zh-CN site without manually editing the Vercel URL. The deployment flow should also verify that the zh-CN preview route is actually reachable before presenting the preview as successful.

Acceptance criteria

• The PR preview comment includes a direct zh-CN preview link, for example the deployment URL with /zh-CN/introduction appended.
• Re-running the preview deployment updates the existing bot preview comment instead of creating duplicate comments.
• The preview deployment verifies at least one zh-CN docs route after deploying, and fails the workflow if the route is unavailable, redirects unexpectedly to the English site, or returns an error page.
• The workflow summary includes the zh-CN preview link alongside the generic preview URL.
• The implementation keeps production deployment behavior unchanged.

Blocked by

None - can start immediately