Survive gitcode API rate limiting when fetching OpenHarmony releases

[linghengqian]

Problem

Linux e2e runs repeatedly failed on every ohos combination with:

lib/ohos.lua:64: OpenHarmony releases are unavailable from
https://gitcode.com/api/v5/repos/CPF-Flutter/flutter_flutter/releases?per_page=100: HTTP 429

The plugin fetched the release list with 3 attempts and a fixed 10s delay,
so any rate-limit window longer than ~20s killed the install. Official-flavor
combos were unaffected (different host).

Fix

lib/ohos.lua: distinguish HTTP 429 from ordinary failures —

• 429 backs off progressively (30s, 60s, 120s, 180s) over up to 5 attempts,
  so installs can ride out short quota windows;
• other errors keep the short 10s delay since there is no window to wait out.

Verification

Exercised the retry logic locally with scripted gitcode responses
(first-try success, 429-then-recover, persistent 429, HTTP 500, non-JSON
body): backoff sequences and attempt budgets behave as described above.
The full proof is the Linux e2e ohos matrix going green on CI.