fix(ci): retry stale beta channel manifest readback

[wgqqqqq]

Summary

After replacing channel-beta/latest.json, GitHub can briefly return the previous CDN object with HTTP 200. This failed the final publication step of run 33976039135 even though all five platform packages and the correct Beta manifest were published. Retry both HTTP failures and unexpected manifest content, with a bounded wait and a clear error if the expected version never appears.

Type and Areas

Bug fix / CI — Desktop release workflow.

Motivation / Impact

Allows Beta publication to tolerate propagation delays. Keeps the existing prerelease flag, Beta promotion policy, and stable update channel behavior.

Verification

• pnpm run check:github-config: 22 passed, 1 skipped because PowerShell is unavailable locally.
• Regression executes the workflow shell with controlled responses: stale content then success, transient HTTP failure, malformed JSON, and persistent stale content that must fail.
• git diff --check: passed.
• Actual upstream release has 30 assets; its manifest and the Beta channel currently both report 1.0.0-beta.1. Stable latest remains v0.2.19.

Reviewer Notes

Only publication verification changes; runtime and remote scenarios are not affected or exercised. The retry stays inline so a workflow can publish packages from an older immutable source tag without requiring a new helper in that tag.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.