[rubysrc2cpg] Retry ruby_ast_gen download on HTTP errors

[max-leuthaeuser]

Motivation

We've seen transient HTTP errors fail the build during rubysrc2cpg's astGenResourceTask, which downloaded the ruby_ast_gen zip via a single-shot urlInputStream + IO.transfer with no retry logic.

Change

Route the download through DownloadHelper.ensureIsAvailable (backed by UrlRetry.downloadWithRetries: 5 attempts, exponential backoff, non-2xx status check) — the same helper already used by joernTypeStubsDlTask and all other frontends' astgen downloads.

Behavior is otherwise unchanged: the download still only happens when the unpacked astgen version doesn't match, and the zip is still deleted after unzipping. Since the URL bookkeeping in .local/source-urls is only written after a successful download, a failed attempt always triggers a fresh retry on the next build.

Verification

• Forced the download path by removing the unpacked version.rb, then ran sbt rubysrc2cpg/astGenResourceTask: download went through ensureIsAvailable ([INFO] downloading ...), unzip + zip cleanup succeeded.
• Re-ran the task: correctly short-circuits with no download once the version matches.
• Audited all other .sbt files for raw download primitives (urlInputStream, IO.transfer, toURL, openStream, curl/wget): no remaining occurrences — every build-time HTTP download in the repo now goes through the retrying helper.