fix(ci): handle GitHub release API failures safely

[Wangnov]

What changed

• add a shared GitHub API helper that retries HTTP 429/5xx failures
• distinguish confirmed HTTP 404 responses from transient or unknown API errors
• make Stable and Beta release publishing fail closed unless absence is confirmed
• add regression coverage for recovered and persistent HTTP 503 responses

Root cause

The release probe and publisher treated any gh exit status of 1 as proof that a Release did not exist. During GitHub API HTTP 503 responses, the workflow therefore attempted to create an already-existing Release and failed with a release with the same tag name already exists.

Validation

• bash -n scripts/*.sh
• actionlint
• all scripts/test-*.sh
• git diff --check